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Population, Resources, Environment: Dimensions of the Human Predicament 

It is clear that the future course of history will be determined by the rates at which people breed 
and die, by the rapidity with which nonrenewable resources are consumed, by the extent and 
speed with which agricultural production can be improved, by the rate at which the 
underdeveloped areas can industrialize, by the rapidity with which we are able to develop new 
resources, as well as by the extent to which we succeed in avoiding future wars. All of these 
factors are interlocked. 

—Harrison Brown, 1954 

Providing people with the ingredients of material wellbeing requires physical resources - land, 
water, energy, and minerals — and the supporting contributions of environmental processes. 
Technology and social organization are the tools with which society transforms physical 
resources and human labor into distributed goods and services. These cultural tools are 
embedded in the fabric of the biological and geophysical environment; they are not independent 
of it. 

As the number of people grows and the amounts of goods and services provided per person 
increase, the associated demands on resources, technology, social organization, and 
environmental processes become more intense and more complicated, and the interactions 
among these factors become increasingly consequential. It is the interactions ~ technology with 
employment, energy with environment, environment and energy with agriculture, food and 
energy with international relations, and so on ~ that generate many of the most vexing aspects of 
civilization's predicament in the last quarter of the twentieth century. 

This book is about that predicament: about its physical underpinnings in the structure of the 
environment and the character of natural resources; about its human dimensions in the size, 
distribution, and economic condition of the world's population; about the impact of that 
population on the ecological systems of Earth and the impact of environmental changes on 
humanity; about the 


technology, economics, politics, and individual behavior that have contributed to the 
predicament; and about the changes that might alleviate it. 


In a world inhabited in the mid- 1970s by a rapidly growing population of more than 4 billion 
people, a massive and widening gap in well-being separates a rich minority from the poor 
majority. The one-third of the world's population that lives in the most heavily industrialized 
nations (commonly termed developed countries — DCs) accounts for 85 percent of the global 
personal income and a like fraction of the annual use of global resources. The people living in 
the less industrialized nations (often called less developed countries -LDCs) must apportion the 
remaining 15 percent of global income and resource use among two-thirds of the world's 
population. The result is an unstable prosperity for the majority of people in the DCs and 
frustrating, crushing poverty for the majority in the LDCs. Millions of the poorest — especially 
infants and children — have starved to death every year for decades; hundreds of millions have 
lived constantly, often consciously, almost always helplessly on the brink of famine and 
epidemic disease, awaiting only some modest quirk of an environment already stretched taut — 
an earthquake, a flood, a drought — to push them over that edge. The 1970s brought an apparent 
increase in such quirks ~ 1972 and 1974 were years of flood, drought, and poor harvests. World 
food reserves plummeted, and millions more human beings were threatened by famine. 
Meanwhile, the entire population continued growing at a rate that would double the number of 
people in the world within 40 years. 

The prosperity of the DCs - awesome by comparison with the poverty of the LDCs — has been 
built on exploitation of the richest soils, the most accessible fossil fuels, and the most 
concentrated mineral deposits of the entire globe — a one-time windfall. As they now struggle to 
maintain and even expand their massive consumption from a resource base of declining quality, 
the DCs by themselves appear to be taxing technology, social organization, and the physical 
environment beyond what they can long sustain. And the LDCs, as they try to follow the same 
path to economic development, find the bridges burned ahead of them. There will be no 
counterpart to the windfall of cheap resources that propelled the DCs into prosperity. A DC-style 
industrialization of the LDCs, based on the expensive resources that remain, is therefore 
probably foredoomed by enormous if not insurmountable economic and environmental obstacles. 

The problems arising from this situation would be formidable even if the world were 
characterized by political stability, no population growth, widespread recognition of civilization's 
dependence on environmental processes, and a universally shared commitment to the task of 
closing the prosperity gap. In the real world, characterized by deep ideological divisions and 
territorial disputes, rapid growth of population and faltering food production, the popular illusion 
that technology has freed society from dependence on the environment, and the determined 
adherence of DC governments to a pattern of economic growth that enlarges existing disparities 
rather than narrowing them, the difficulties are enormously multiplied. 


It takes water and steel to produce fuel, fuel and water to produce steel, fuel and water and steel 
to produce food and fiber, and so on. The higher the level of industrialization in a society, the 
more intimate and demanding are the interconnections among resources. Agriculture in the 
United States, Europe, and Japan, for example, uses far more fuel, steel, and mineral fertilizers 
per unit of food produced than does agriculture in India or Indonesia. The interconnections 
among resources also become more intense as the quality of the resource base diminishes; the 

amount of fuel and metal that must be invested in securing more fuel and metal increases as 
exhaustion of rich deposits forces operations deeper, farther afield, and into leaner ores. 

These tightening physical links among resources have their counterparts in economic and 
organizational ef- 


fects. Scarcity or rising prices of one commodity generate scarcity and rising prices of others, 
thus contributing both directly and indirectly to inflation and often to unemployment. Massive 
diversion of investment capital and technical resources to meet the crisis of the moment — 
attempting to compensate for lack of foresight with brute force applied too late — weakens a 
system elsewhere and thus promotes crises in other sectors later. Apparent solutions seized in 
haste and ignorance cut off options that may be sorely missed when future predicaments arise. 

International aspects add to the complexity — and the dangers ~ of these interactions. Money 
pours across international boundaries, collecting in those parts of the world where rich deposits 
of essential resources still remain. In resource-importing DCs, the pressure to redress the 
balance-of-payments imbalance becomes the dominant determinant of what is exported, 
subverting other goals. Resources of indeterminate ownership, such as fish stocks in oceans and 
seabed minerals, become the focus of international disputes and unregulated exploitation. And 
foreign policies bend and even reverse themselves to accommodate the perceived physical needs. 

The interactions of resources, economics, and politics were displayed with compelling clarity in 
the worldwide petroleum squeeze of the mid-1970s. The consequences of a slowdown in the 
growth of petroleum production in the principal producing countries, accompanied by almost a 
quadrupling of the world market price, reverberated through all sectors of economic activity in 
DCs and LDCs alike. The prices of gasoline, jet fuel, heating oil, and electricity soared, 
contributing directly to inflation. Increased demand for petroleum substitutes such as coal drove 
the prices of those commodities up, thereby contributing indirectly to inflation, as well. 
Shortages of materials and services that are particularly dependent on petroleum for their 
production or delivery quickly materialized, feeding inflation and unemployment still further. 

The impact of the energy crisis was especially severe on the already precarious world food 
situation. Indeed, rising food prices, following the poor harvests of 1972, were a major factor in 
the decision by the Organization of Petroleum Exporting Countries (OPEC) to raise oil prices. 
The strong and growing dependence of agriculture on energy ~ especially petroleum — soon 
made itself felt worldwide as an important contributor to further increases in food prices. This 
was especially so in developed countries (where agriculture is most highly mechanized), a few of 
which are the main sources of the exportable food supplies that determine world market prices. 
The largest exporter, the United States, has gratefully responded to increased foreign demand for 
its food exports, which has helped to pay for the nation's increasingly expensive oil imports (but 
which has also contributed to raising domestic food prices). This phenomenon ~ the need to sell 
food abroad to pay for oil ~ may have assured continued high-quality diets for nations like Japan 
that can still afford to buy, but it also reduced the amount of uncommitted food reserves in the 

DCs that would be available to alleviate famines in LDCs too poor to buy the food they needed 
on the world market. 

The most obvious consequence of the 1973 petroleum embargo and price rise on international 
political affairs was one intended by the oil-producing Arab states ~ a sudden diminution of DC 
support for the Israeli position in Middle East territorial disputes. Some less obvious effects, 
however, may in the long run be more significant. The United States and several other DCs 
export military hardware as a major source of the foreign exchange they need to pay for imported 
raw materials; and the intensity with which the arms exporters hustle their wares in the 
international market is increasing as the oil-related balance-of-payments problem worsens. The 
result has been to support a spiraling arms race in LDCs, which is both a pathetic diversion of 
funds needed there to raise the standards of living and a profoundly destabilizing force operating 
against world peace. The export of nuclear reactors, likewise encouraged by the DCs' need to pay 
for imported raw materials, may also have disastrous effects. Although intended for production 
of electricity, these reactors also provide their LDC recipients with the materials needed to 
manufacture nuclear bombs. India's nuclear explosion in 1974 demonstrated for any remaining 
doubters that the spread of reactors can mean the spread of nuclear weapons. And it must be 
assumed that the distribution of such weapons into more 


and more hands ~ and into some of the most politically troubled regions of the world ~ greatly 
increases the chances that they will be used. 


The relationship connecting technology, environment, and well-being would constitute a deep 
dilemma for civilization, even in the absence of the economic and political complexities just 
described. Simply stated, the dilemma is this: while the intelligent application of technology 
fosters human well-being directly, a reducible but not removable burden of environmental 
disruption by that technology undermines well-being. This negative burden includes the direct 
effects of technology's accidents and effluents on human life and health; the direct impact of 
accidents, effluents, preemption of resources, and transformation of landscapes on economic 
goods and services; and technology's indirect adverse impact on well-being via disruption of the 
vital services supplied to humanity by natural ecological systems. These free services — 
including, among others, the assimilation and recycling of wastes — are essential to human health 
and economic productivity. It is clearly possible to reach a point where the gain in well-being 
associated with (for example) producing more material goods does not compensate for the loss in 
well-being caused by the environmental damage generated by the technology that produced the 
goods. Beyond that point, pursuing increased prosperity merely by intensifying technological 
activity is counterproductive. Many people would argue that the United States has already passed 
that pivotal point. 

The turning point where environmental costs begin to exceed economic benefits can be pushed 
back somewhat by using technologies that cause the least possible environmental damage. It 
cannot be pushed back indefinitely, however, because no technology can be completely free of 

environmental impact. This flat statement ~ implying that continued expansion of any 
technology will eventually lead to environmental costs exceeding its benefits ~ is true on 
fundamental physical grounds. It means that environmental constraints will ultimately limit 
economic production — the product of the number of people and the amount of economic goods 
and services that each person commands — if nothing else limits it first. 

Despite increased scientific attention to environmental problems in recent years, most of the 
potentially serious threats are only sketchily understood. For many such problems, it cannot be 
stated with assurance whether serious damage is imminent, many decades away, or indeed 
already occurring — with the full consequences yet to manifest themselves. Much remains to be 
learned, for example, about the causation of cancer and genetic damage by low concentrations of 
nearly ubiquitous environmental contaminants, either alone or in combination: pesticide residues, 
combustion products, heavy metals, plasticizers, food additives, prescription and nonprescription 
drugs, and innumerable others. 

The most serious and imminent peril of all may well prove to be civilization's interference with 
the "public service" functions of environmental processes. Agricultural production in a world 
already on the brink of famine depends intimately on the absence of major fluctuations in 
climate, on the chemical balances in soil and surface water that are governed by biological and 
geochemical nutrient cycles, on naturally occurring organisms for pollination of crops, and on 
the control of potential crop pests and diseases by natural enemies and environmental conditions. 
Agents of human disease and the vectors that transport those agents are also regulated in large 
part by climate, by environmental chemistry, and by natural enemies. Ocean fish stocks — an 
important source of food protein ~ depend critically on the biological integrity of the estuaries 
and onshore waters that serve as spawning grounds and nurseries. The environmental processes 
that regulate climate, build and preserve soils, cycle nutrients, control pests and parasites, help to 
propagate crops, and maintain the quality of the ocean habitat, are therefore absolutely essential 
to human well-being. Unfortunately, the side effects of technology are systematically 
diminishing the capacity of the environment to perform these essential services at the same time 
that the growth of population and the desire for greater affluence per capita are creating greater 
demand for them. 


Nor can the public-service functions of the environment be safely replaced by technology if 
technology destroys them. Often the foresight, scientific knowledge, and technological skill that 
would be required to perform this substitution just do not exist. Where they do exist, the 
economic cost of an operation on the needed scale is almost invariably too high; and where the 
economic cost at first seems acceptable, the attempt to replace environmental services with 
technological ones initiates a vicious circle: the side effects of the additional technology disrupt 
more environmental services, which must be replaced with still more technology, and so on. 


The foregoing brief survey of the dimensions of the human predicament suggests a discouraging 
outlook for the coming decades. A continuing set of interlocking shortages is likely ~ food, 

energy, raw materials — generating not only direct increases in human suffering and deprivation, 
but also increased political tension and (perversely) increased availability of the military 
wherewithal for LDCs to relieve their frustrations aggressively. Resort to military action is 
possible, not only in the case of LDCs unwilling to suffer quietly, but, with equal or greater 
likelihood, in the case of industrial powers whose high standard of living is threatened by denial 
of external resources. The probability that conflicts of any origin will escalate into an exchange 
of nuclear weapons, moreover, can hardly fail to be greater in 1985's world of perhaps fifteen or 
twenty nucleararmed nations than it has been in the recent world of five. 

The growth of population — very rapid in the LDCs, but not negligible in most DCs, either — will 
continue to compound the predicament by increasing pressure on resources, on the environment, 
and on human institutions. Rapid expansion of old technologies and the hasty deployment of new 
ones, stimulated by the pressure of more people wanting more goods and services per person, 
will surely lead to some major mistakes — actions whose environmental or social impacts erode 
well-being far more than their economic results enhance it. 

This gloomy prognosis, to which a growing number of scholars and other observers reluctantly 
subscribes, has motivated a host of proposals for organized evasive action: population control, 
limitation of material consumption, redistribution of wealth, transitions to technologies that are 
environmentally and socially less disruptive than today's, and movement toward some kind of 
world government, among others. Implementation of such action would itself have some 
significant economic and social costs, and it would require an unprecedented international 
consensus and exercise of public will to succeed. That no such consensus is even in sight has 
been illustrated clearly by the diplomatic squabbling and nonperformance that have characterized 
major international conferences on the environment, population, and resources, such as the 
Stockholm conference on the environment in 1972, the Bucharest Conference on World 
Population in 1974, the Rome Food Conference in 1974, and the Conferences on the Law of the 
Sea in the early 1970s. 

One reason for the lack of consensus is the existence and continuing wide appeal of a quite 
different view of civilization's prospects. This view holds that humanity sits on the edge of a 
technological golden age; that cheap energy and the vast stores of minerals available at low 
concentration in seawater and common rock will permit technology to produce more of 
everything and to do it cheaply enough that the poor can become prosperous; and that all this can 
be accomplished even in the face of continued population growth. In this view ~ one might call 
it the cornucopian vision — the benefits of expanded technology almost always greatly outweigh 
the environmental and social costs, which are perceived as having been greatly exaggerated, 
anyway. The vision holds that industrial civilization is very much on the right track, and that 
more of the same — continued economic growth -with perhaps a little luck in avoiding a major 
war are all that is needed to usher in an era of permanent, worldwide prosperity. - 

Outstanding proponents of this view include British economist Wilfred Beckerman ( Two 
cheers for the affluent society, St. Martin's Press, London, 1974): British physicist John 
Maddox ( The doomsday syndrome, McGraw-Hill, New York, 1972); and American 
futurologist Herman Ka hn ( The next 200 years, with William Brown and Leon Martel, 
William Morrow, New York, 1976). 


Which view of civilization's prospects is more accurate is a question that deserves everyone's 
scrutiny. It cannot be decided merely by counting the "experts" who speak out on either side and 
then weighing their credentials. Rather, the arguments must be considered in detail ~ examined, 
dissected, subjected to the test of comparison with the evidence around us. This is an ambitious 
task, for the issues encompass elements of physics, chemistry, biology, demography, economics, 
politics, agriculture, and a good many other fields as well. One must grapple with the arithmetic 
of growth, the machinery of important environmental processes, the geology of mineral 
resources, the potential and limitations of technology, and the sociology of change. It is 
necessary to ponder the benefits and shortcomings of proposed alternatives as well as those of 
the status quo; and it is important to ask where burdens of proof should lie. 

Does civilization risk more if the cornucopians prevail and they are wrong? or if the pessimists 
prevail and they are wrong? Could an intermediate position be correct, or are perhaps even the 
pessimists too optimistic? What is the most prudent course in the face of uncertainty? Making 
such evaluations is, of course, a continuing process, subject to revision as new arguments, 
proposals, and evidence come to light. What is provided in the following chapters is a starting 
point: a presentation of essential principles, relevant data, and (we think) plausible analyses that 
bear on the predicament of humanity. 

Recommended for Further Reading 

Brown, Harrison. 1954. The challenge of man's future. Viking, New York. A landmark in the 
literature of the human predicament, perceptively elucidating the interactions of population, 
technology, and resources. Brown alerted a generation of scholars and policy-makers to the 
seriousness of impending problems and has worked tirelessly in national and international 
scientific circles to mobilize the talent to help solve them. 

Ehrlich, Paul R.; Anne H. Ehrlich; and John P. Holdren. 1973. Human ecology. W. H. Freeman 
and Company, San Francisco. A predecessor of this book, providing capsule introductions to 
many of the topics expanded upon here. 

National Academy of Sciences-National Research Council. 1969. Resources and man. Report of 
the Committee on Resources and Man of the National Academy of Sciences. W. H. Freeman and 
Company, San Francisco. A sober look at population, food, energy, minerals, and environment, 
together with forthright policy recommendations, by a group of experts both eminent and 

Osborn, Fairfield. 1948. Our plundered planet. Little, Brown, Boston. A prescient early plea for 
an integrated approach to the interlocking problems of population, resources, and environment. 


Section I 

Natural Processes and Human Weil-Being 


There is one ocean, with coves having many names; 
a single sea of atmosphere, with no coves at all; 
a thin miracle of soil, alive and giving life; 
a last planet; and there is no spare. 

— David R. Brower 

Prehistoric human beings earned a living from their surroundings in much the same way as many 
other animals did (and do): they sought out and preyed upon other organisms ~ edible plants and 
killable animals — that shared the environment, they drank water where they found it, and they 
took shelter in trees and caves. The geographical distribution, size, and well-being of the human 
population under these circumstances were influenced very strongly by the characteristics of the 
natural environment — patterns of hot and cold, wet and dry, steep and flat, lush and sparse. 
These patterns of climate, topography, flora, and fauna were in turn the products of millions of 
centuries of interaction among natural geophysical and biological processes — continental drift, 
mountain-building, erosion, sedimentation, the advance and retreat of glaciers, the rise and fall of 
the oceans, and the evolution and extinction of various kinds of organisms. 

Before the advent of agriculture, most of the natural processes and systems that so strongly 
influenced the human condition were themselves not much influenced by what humans did; the 
interaction was largely a one-way street. Fire and stone implements apparently were used even 
by the forerunners of Homo sapiens well over a million years ago, and the wielders of those early 
tools certainly used them to affect their local environments, both intentionally and inadvertently. 
But it was agriculture, which began about 10,000 years ago, that marked the sharpest transition 
in the capacity of Homo sapiens to influence the physical environment on a large scale. From 
that beginning, which permitted specialization and a scale of social organization not possible 
among hunter-gatherers, arose a long series of technological and social developments that 
produced the agricultural and industrial civilizations that today virtually cover the globe. 


That civilizations have transformed many aspects of the physical environment is beyond dispute. 
Indeed, the prevalence of "man-made" environments (such as cities) and intensively managed 
ones (such as wheat fields) makes it all too easy to suppose that such technological environments 
are now the only ones that matter. This supposition, which one could call the humanity-is-now- 
the-master-of-nature hypothesis, is partly responsible for the widespread underestimation of the 
seriousness of environmental problems. Regardless of such beliefs, civilizations and their 
technological environments continue to be embedded, as they always have been, in a larger 
nontechnological environment. This larger environment provides the raw materials with which 
technology must work, and its characteristics together with those of technology define the limits 
of what is possible at any given time. 

Technology and natural environmental processes together - not technology alone — have 
permitted the human population and its material consumption to reach their present levels. Today 
human beings continue to depend on the nontechnological environment for a variety of services 
that technology not only cannot replace, but without which many technological processes 
themselves would be nonfunctional or unaffordable. Many of these services are mentioned in 
Chapter 1 — the regulation of climate, the management of soil and surface water, the 
environmental chemistry of nutrient cycling and control of contaminants, and the regulation of 
pests and disease, among others. To understand the nature and extent of humanity's dependence 
on these services, and the degree of their vulnerability to disruption by mismanagement and 
overload, one must investigate in detail the character of the physical and biological processes 
that provide them. We do so in the next three chapters: "The Physical World," discussing basic 
geological processes, the hydrologic cycle, and climate; "Nutrient Cycles," describing the major 
pathways by which carbon, nitrogen, phosphorus, and other essential nutrients move from the 
living to the nonliving environment and back again; and "Populations and Ecosystems," an 
introduction to the principles of population biology and ecology essential to an understanding of 
the structure and functions of biological communities. The details of the actual threats to these 
processes posed by the activities of contemporary human beings are taken up in Chapter 1 1 . 


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The Physical World 

All the rivers run into the sea; yet the sea is not full. 


The biosphere is that part of Earth where life exists. In vertical dimension it extends from the 
deepest trenches in the ocean floor, more than 1 1,000 meters (36,000 feet) ^below sea level, to at 
least 10,000 meters (m) above sea level, where spores (reproductive cells) of bacteria and fungi 
can be found floating free in the atmosphere. By far most living things — most of which depend 
directly or indirectly on the capture of solar energy by photosynthesis in plants and certain 
bacteria — exist in the narrower region extending from the limit of penetration of sunlight in the 
clearest oceans, less than 200 meters from the surface, to the highest value of the permanent 
snow line in tropical and subtropical mountain ranges — about 6000 meters, or 20,000 feet. ( 
Everest, the highest mountain, rises almost 8900 meters above sea level.) By any definition, the 
biosphere is as a mere film in thickness compared to the size of the ball of rock on which it sits — 
about like the skin of an apple, in fact. The radius of Earth is about 6370 kilometers (km), or 
4000 miles (mi). 

Of course, conditions within the thin envelope of the biosphere are influenced by physical 
processes taking place far outside it: by the energy emitted by the sun, 150 

Throughout this book physical dimensions are given in metric units, sometimes accompanied 
by the English equivalent to ease the transition for readers not completely accustomed to the 
metric system. For more precise conversion factors, see the tables inside the covers of the 


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FIGURE 2-1 Vertical structure of the physical world. (Scales are greatly distorted.) 

million kilometers away; by the tides originating in the relative motion and positions of Earth, 
sun, and moon (the distance from Earth to the moon averages about 380,000 km); by the 
presence of gases 20 to 400 km high in the atmosphere that screen out harmful components of 
incoming solar energy; by the constitution and structure of Earth's crust (to 40 km deep), which 
govern the availability of mineral nutrients at the surface and of metallic ores accessible to 
industrial civilization; by the behavior of the solid but plastically flowing _mantle (to 2900 km 
deep) on which the crustal plates "float" and move laterally; and by the motion of Earth's molten 
core (2900 to 5200 km deep), which produces the magnetic field that protects the planet's surface 
from bombardment by energetic, electrically charged particles from space. The vertical structure 
of Earth's atmosphere, surface, and interior is illustrated in Figure 2-1 . The terminology used 
there for the various vertical divisions is explained in the following text, where the character of 
the atmosphere and Earth's interior are taken up in more detail. 

In horizontal extent the biosphere covers the globe, although in the hottest deserts and coldest 
polar regions — as at the highest elevations ~ usually only dormant spores can be found. Earth's 
total surface area amounts to 510 million square kilometers (about 197 million mi ), of which 71 
percent is ocean and 29 percent land (see Table 2-1). The mass of all living organisms on Earth 
amounts to about 5 trillion metric tons, _threefourths of which consists of water. Under the 
reasonable assumption that living matter has about the same density as water [1 gram per cubic 
centimeter (lg/cm )], this would mean that the living part of the biosphere was equivalent to a 
layer of material only I centimeter thick, covering the globe. (The range concealed in this 
average is from 0.0002 g of living material for each square centimeter of surface in the open 
ocean to 15 g or more for each square centimeter of surface in a tropical forest.) 

TABLE 2-1 Surface Areas of the Globe 



of category 

(million km 2 ) 





Pacific Ocean - 46 


Atlantic Ocean - 23 


Indian Ocean - 18 

Arctic Ocean - 4 

Mean extent of sea ice ( Arctic, South Atlantic, Pacific, 
and Indian) 



Eurasia 36 

Africa 20 

North and Central America 16 

South America 12 

Antarctica 10 

Oceania 6 

Ice-covered land 10 

Percentage Area 

of category (million km 2 ) 

Terrain more than 3000 meters high 5 

Lakes and rivers 1 

Mean extent of sea ice has been subtracted. 
Source: Strahler and Strahler, Environmental geoscience. 

Plastic flow in a solid refers to continuous deformation in any direction without 


A metric ton (MT) equals 1000 kilograms (kg), or 2205 pounds. One trillion (in 

1 9 

scientific notation, 10 ) equals 1000 billion, or 1,000,000 million. 

TABLE 2-2 Masses of Constituents of the Physical World 

Constituent (trillion MT) 

Living organisms (including water content) 8 

Liquid fresh water, on surface 126 

Atmosphere 5,140 

Ice 30,000 

Salts dissolved in oceans 49,000 

Oceans 1,420,000 

Earth's crust (average depth, 17 km) 24,000,000 

Earth (total) 6,000,000,000 

The relative masses of various constituents of the physical world are shown in Table 2-2. 

To study the processes that operate in any subdivision of the physical world ~ atmosphere, 
biosphere, Earth's crust — one must know something about energy: what it is, how it behaves, 
how it is measured. For whenever and wherever anything is happening, energy in some form is 
involved; it is in many respects the basic currency of the physical world. An introduction to 
energy and the related concepts of work and power, along with the units in which these 
quantities can be measured, is provided in Box 2-1. Some feeling for how much energy is stored 
in and flows between various parts of the physical world is conveyed in Table 2-3. 


The outermost layer of Earth's solid surface is called the crust. It ranges in thickness from about 
6 kilometers beneath the ocean floor to as much as 75 kilometers below the largest mountain 
ranges. In essence, the crust floats on the denser mantle beneath it. (As is elaborated below, the 
crust and mantle are differentiated by the different compositions and densities of the rock they 
comprise.) As with icebergs on the sea, the more crust extends above the surface (as in a 
mountain range), the more bulk is hidden below. ^_This situation is made possible by the 
existence of a soft, yielding layer called the asthenosphere in the middle of the underlying 

mantle. This layer's strength is low because the rock is near its melting point. The combination of 
the crust and the hard upper layer of the mantle is called the lithosphere, a term sometimes also 
employed in a more general sense to mean the entire solid part of Earth. Below the mantle, 
between the depths of 2900 and 5200 kilometers, lies Earth's molten outer core. This core 
consists largely of liquid iron (with some nickel) at a temperature of perhaps 2500° C; its 
properties and motion produce Earth's magnetic field. 

TABLE 2-3 Energy Flow and Storage in the Physical World 

Energy or power 

STORAGE Trillion MJ 

Energy released in a large volcanic eruption 100 

Chemical energy stored in all living organisms 30,000 

Energy released in a large earthquake 100,000 

Chemical energy stored in dead organic matter 100,000 

Heat stored in atmosphere 1,300,000 

Kinetic energy of Earth's rotation on its axis 250,000,000,000 

FLOWS Million Mw 

Tides 3 

Heat flow from Earth's interior 32 

Conversion of sunlight to chemical energy in photosynthesis 1 00 

Conversion of sunlight to energy of motion of atmosphere 1 ,000 

Sunlight striking top of atmosphere 1 72,000 

4 For more detailed treatment of this point and others in this section, see F. Press and R. Siever, 
Earth; and A. N. Strahler and A. H. Strahler, Environmental geoscience. 

BOX 2-1 Work, Energy, and Power: Definitions, Disguises, and Units 

Work is the application of a force through a distance. Energy is stored work. Power is the rate of 
flow of energy, or the rate at which work is done. All these concepts are more easily understood 
with the help of examples and some elaboration. 

Work — force multiplied by distance ~ is done when a weight is lifted against the force of 
gravity (as with water carried upward in the atmosphere in the course of the hydrologic cycle), 
when mass is accelerated against the resistance of inertia (as with waves whipped up on the 
ocean by the wind) or when a body is pushed or pulled through a resisting medium (as with an 
aircraft moving through the atmosphere or a plow cutting through a field). The presence of 
distance in the concept of work means that work is done only if there is motion ~ if you push on 
a stalled car and it doesn't budge, there is a force, but there is no work because there is no 

The foregoing are examples of mechanical work — work involving the bulk (or macroscopic) 
motion of agglomerations of molecules. There are also various forms of microscopic work, such 
as chemical work and electrical work, which involve forces and motions on the scale of 
individual molecules, atoms, and electrons. To heat a substance is to do a form of microscopic 
work in which the individual molecules of the substance are made to move more rapidly about in 
all directions, without any bulk motion taking place. The demonstration that all these different 
manifestations of work are fundamentally the same can be found in treatises on physics and 
chemistry. - 

If work has many guises, so must energy, which is only stored work. Work stored as the motion 
of a macroscopic object (for example, a speeding automobile or the Earth spinning on its axis) is 
called mechanical energy or kinetic energy. The latter term may be applied as well to the energy 
of motion of microscopic objects (such as, molecules, electrons). Work stored as the disordered 
motion of molecules — that is, rotation, vibration, and random linear motion not associated with 
bulk motion of the substance — is called thermal energy or sensible heat or (more commonly) 
just heat. Note that temperature and heat are not the same. Temperature is a measure of the 
intensity of the disordered motion of a typical molecule in a substance; the heat in a substance is 
the sum of the energies stored in the disordered motion of all its molecules. (The relation 
between temperature and energy is developed further in Box 2-3.) 

Kinetic energy means something is happening; that is, the work is stored as motion. Potential or 
latent energy means something is "waiting" to happen. That is, the work is stored in the position 
or structure of objects that are subject to a force and a restraint; the force provides the potential 
for converting position or structure into kinetic energy, and the restraint is what keeps this from 
happening (at least temporarily). Each kind of potential energy is associated with a specific kind 
of force. Gravitational potential energy (an avalanche waiting to fall) and electrical potential 
energy (oppositely charged clouds waiting for a lightning stroke to surge between them) are 
associated with forces that can act between objects at large distances. Chemical potential energy 
(gasoline waiting to be burned, carbohydrate waiting to be metabolized) is associated with the 
forces that hold atoms together in molecules — that is, with chemical bonds. Nuclear potential 
energy is due to the forces that hold protons and neutrons together in the nucleus — the so-called 
strong force. Latent heat of vaporization (water vapor waiting to condense into liquid, 
whereupon the latent heat will be converted to sensible heat) and latent heat of fusion (liquid 
waiting to freeze into a solid, with the same result) are associated with the electrical forces 
between molecules in liquids and solids. The idea that potential energy is something "waiting" to 
happen needs only to be tempered by recognition that sometimes it can be a long wait — the 
chemical potential energy in a piece of coal buried in Earth's crust, for example, may already 
have waited a hundred million years. 

Electromagnetic radiation is a form of energy that does not fall neatly into any of the categories 
we have mentioned so far. It is characterized not {Continued) 

See, for example, R. Feynmann, R. Leighton, and M. Sands, The Feymann lectures on 
physics, Addison-Wesley, Reading, Mass., 1965. 

Box 2-1 (Continued) in terms of the motion or position or structure of objects but in terms of the 

motion of electric and magnetic forces. Light (visible electromagnetic radiation), radio waves, 

thermal (infrared) radiation, and X-rays are all closely related varieties of this particular form of 
energy. (See also Box 2-4.) 

Albert Einstein theorized, and many experiments have subsequently verified, that any change in 
the energy associated with an object (regardless of the form of the energy) is accompanied by a 
corresponding change in mass. In this sense, mass and energy are equivalent and 
interchangeable, the formal expression of equivalence being Einstein's famous formula E = mc . 
(Here E denotes energy, m mass, and c the speed of light.) Because a small amount of mass is 
equivalent to a very large quantity of energy, a change in mass is only detectable when the 
change in energy is very large ~ as, for example, in nuclear explosions. 

Different professions use a bewildering array of units for counting work and energy. The metric 
system is prevailing, but so gradually that the literature of energy and environmental sciences 
will be littered for years to come by a needless profusion of archaic units. Since work has the 
dimensions of force times distance, and all energy can be thought of as stored work, it should be 
apparent that a single unit will suffice for all forms of energy and work. The most logical one is 
the joule (J), which is exactly the amount of work done in exerting the basic metric unit of force, 
1 newton (N), over the basic metric unit of distance, 1 meter. ^_We shall use the joule and its 
multiples, the kilojoule (1000 J, or kJ) and the megajoule (1,000,000 J, or MJ), throughout this 

Our only exception to the use of the joule is a concession to the enormous inertia of custom in 
the field of nutrition, where we reluctantly employ the kilocalorie (kcal). A kilocalorie is 
approximately the amount of thermal energy needed to raise the temperature of 1 kg of water by 
1 degree Celsius (1° C); ^Jhis unit is often confusingly written as "calorie" in discussions of 
nutrition. Running the bodily machinery of an average adult human being uses about 2500 kcal — 
about 10,000 kJ ~ per day.) 

Besides the erg (1 ten-millionth of ajoule) and the calorie (1 thousandth of akcal), the unit of 
energy most likely to be encountered by the reader elsewhere is the British thermal unit (Btu), 
which is approximately the amount of thermal energy needed to raise I pound of water by 1 
degree Fahrenheit (1° F). A Btu is roughly a kilojoule. 

In many applications one must consider not only amounts of energy but the rate at which energy 
flows or is used. The rate of energy flow or use is power. Useful power is the rate at which the 
flow of energy actually accomplishes work. The units for power are units of energy divided by 
units of time — for example, British thermal units per hour, kilocalories per minute, and joules 
per second. One joule per second is a watt (w). A kilowatt (kw) is 1000 watts, and a megawatt 
(Mw) is 1,000,000 watts; these are the units we use for power in this book. These units are 
perfectly applicable to flows of nonelectric as well as electric energy, although you may be 
accustomed to them only in the context of electricity. Similarly, the kilowatt hour (kwh), 
denoting the amount of energy that flows in an hour if the rate (power) is 1 kilowatt, makes sense 
as a unit of energy outside the electrical context. A kilowatt hour is 3600 kilojoules. For 
example, we can speak of an automobile using energy (in this case, chemical energy stored in 

gasoline) at a rate of 100 kilowatts (100 kilojoules of chemical energy per second). In an hour of 
steady driving at this rate of fuel consumption, the automobile uses 100 kJ/sec multiplied by 
3600 sec (the number of seconds in an hour) or 360,000 kJ. The same quantity of energy used in 
a jumbo jetliner would produce a much larger power -say, 180,000 kw — for a much shorter time 
(2 sec). 

A complete set of conversion factors for units of energy and power appears just inside the covers 
of this book. 

A physicist might object at this point that it isn't always useful, or even possible, to 
discriminate between objects and fields of electric and magnetic force. This level of 
technicality will not be needed in this book. 

^The units of force are units of mass multiplied by units of acceleration. One newton is a mass 
of 1 kg times an acceleration of 1 m per second (sec) per second (1 N = 1 kgm/sec ). One 
joule equals 1 newton-meter (1 J = 1 N-m = 1 kgm /sec ). 
^Zero and 100 on the Celsius, or centigrade, scale of temperature correspond to the freezing 
point and the boiling point of pure water. The conversion between Celsius (C) and Fahrenheit 
(F) is: degrees F = 1.8 x degrees C + 32. 

Within the molten outer core is a solid inner core, also composed of iron and nickel, under 
enormous pressure. 

Many of the characteristics of Earth's solid surface are the result of the operation of tectonic 
processes— the motion of great solid segments of the lithosphere, called plates, which slide about 
over the plastically flowing asthenosphere at a rate of a few centimeters per year. Operating over 
hundreds of millions of years, such motions have apparently produced displacements of 
thousands of kilometers. The now widely accepted theory of continental drift holds that the 
present arrangement of the continents arose in this way, beginning with the breakup of the single 
supercontinent Pangaea about 200 million years ago. - 

Some of the main tectonic processes, as they continue to work today, are illustrated in Figure 2- 
2. At divergent plate boundaries on the ocean floor, such as the East Pacific and Mid-Atlantic 
Ridges, adjacent plates move apart and new crust is created in the gap by magma (molten rock 
which rises from below and then solidifies. This phenomenon is called seafloor spreading. At 
convergent plate boundaries, as along the western edge of South America, one plate may be 
driven beneath the other into the asthenosphere. Heat generated by the friction in these 
subduction zones melts some of the crustal rock to produce magma, which rises to feed volcanic 
activity at the surface. Deep-sea trenches, steep mountain ranges, and powerful earthquakes are 
other characteristics of these zones of violent collision between plates. At a third type of 
interface between plates, the plates slide past each other, moving parallel to the boundary. These 
parallel-plate boundaries are characterized by earthquakes with large surface displacements; the 
San Andreas Fault, which produced the great San Francisco earthquake of 1906, marks such a 
boundary. The principal plate boundaries are indicated on the map in Figure 2-3 . 

Many other geophysical processes operate simultaneously with the tectonic motions described 
above to govern the shape and composition of Earth's crust. These processes include mountain- 
building by uplifting of the crust, the wearing-away of exposed rock surfaces by the actions of 
wind, rain, ice, and chemical processes (together these effects are called weathering), the 

L*_J B*hAc cruti And tipper mart* 
>"*£}» Or**)* cruM 

~2 5**mwl* and MttrnenlMy rock 

— ** Ows(» Eim»ctipn nl pi«» mOHqn 





FIGURE 2-2 Tectonic processes and the Earth's surface. (From P. A. Rona, "Plate tectonics and 
mineral resources", Scientific American, July, 1973.) 

See J. Tuzo Wilson, ed., Continents adrift. 

FIGURE 2-3 Six principal tectonic plates of the lithosphere. 

of particles of rock and soil by water and wind (erosion), and the formation and transformation 
of new rocks from sedimentary material. The way in which these processes are linked together to 
produce the principal geological cycles is represented schematically in Figure 2-4 . 

Rock that is exposed at the surface of the crust is gradually weathered away by physical and 
chemical processes. The resulting particles are some of the raw materials for new soil (the 
formation of which also requires the action of living organisms), and some of the chemicals 
liberated from the rock become available to the biosphere as nutrients (see Chapter 3). Although 
the rock particles may sometimes be carried uphill by wind and ice, the predominant motion is 
downhill with the flow of water. Thus it happens over geologic spans of time (hundreds of 
thousands to millions of years) that large amounts of material are removed from the exposed 
rocky crust at high elevations and deposited on the lowlands and on the ocean floors. The 
accumulating weight of these sediments, consisting ultimately not only of rock fragments but 
also of dead plant and animal matter and chemicals precipitated out of seawater, contributes to 
sinking of the underlying crust and upper mantle. (Tectonic subsidence, the result of large-scale 
crustal motions, is more important in this sinking phenomenon than is the local accumulation of 
sediment weight, however. _J Simultaneously, crustal rise, or uplifting, under the lightened 
regions restores some of the loss of elevation produced by weathering and erosion. This process 
of sinking and uplifting is made possible by the capacity of the dense but soft (almost molten) 
asthenosphere to be deformed and, indeed, to flow. The folding 

Press and Siever, Earth, p. 479. 








t i f 










t " 










uer amorphic 










FIGURE 2-4 Geologic cycles. Intrusive igneous rocks are those that solidify from magma before 

reaching the surface, in contrast to extrusive igneous rocks (lava). Most sedimentation 

(deposition of sediments) takes place on the ocean floor. The time for material to complete a 

cycle is typically tens of millions to hundreds of millions of years. 

and buckling of Earth's crust, which has produced much of the varied topography we see, is the 
combined result of the sinking-uplifting phenomenon just described and the continuous collision 
of the great lithospheric plates. 

As layers of sediment become more deeply buried, they are subjected to temperatures and 
pressures high enough to initiate chemical and physical changes that transform the sediments 
into rock (called sedimentary rocks). Among the rocks formed in this way are shale, sandstone, 
limestone, and dolomite. Under some conditions, such as the particularly energetic geological 
environment where tectonic plates collide, further transformations under the action of heat and 
pressure produce metamorphic rocks, among which are slate and marble. The most abundant 
rocks in Earth's crust, however, are igneous rocks -- those formed by the cooling and 
solidification of magma. Repeated local melting, migration, and resolidification of the rock in 
Earth's crust and upper mantle have led over the eons to a general stratification, with the densest 
material on the bottom and less dense material above. Thus, the upper layer of the continental 
crust consists largely of granitic igneous rocks—rocks rich in the relatively light elements silicon 
and aluminum. The oceanic crust and the lower layer of the 


continental crust ( Figure 2-1 ) consist mainly of basaltic igneous rocks—somewhat denser 
material, containing substantial amounts of iron in addition to the lighter elements. The mantle 
below is olivine igneous rock, richer yet in iron and therefore denser than the overlying crust. 

The average elemental composition of Earth's crust is given in Table 2-4 . The predominance of 
the light elements is apparent: of the ten most abundant elements—accounting for 99 percent of 
the mass of the crust— only iron has an atomic number above 25. The crust comprises only about 
0.4 percent of the mass of Earth, however. Essentially all the rest resides in the denser and vastly 
thicker mantle and core ( Table 2-2 ). The composition of the entire planet ( Table 2-5 ), reflects 
the predominance of iron in those inner layers. Of interest is that carbon, the basic building block 
of living 

TABLE 2-4 Average Composition of Earth's Crust 


Atomic number 































Source: Brian J. 

Skinner, Earth resources. 

TABLE 2-5 Average Composition of Earth (Overall) 


Atomic number 































Percentage by weight 

Percentage by weight 

Source: Brian Mason, Principles of geochemistry . 

material, is not among the most abundant elements (it ranks fourteenth in crustal abundance, at 
0.032 percent). 

The energy that drives the great geological cycles has two distinct origins. Those parts of the 
cycles that take place on the surface—weathering, the formation of soil, erosion, the production 
of plant and animal matter that contributes to sediments—are powered by solar energy and its 
derivatives, wind and falling water. (The character of these energies is examined more closely 
later in this chapter.) The remaining geophysical processes (for example, the production and 
migration of magma and the inexorable motions of the tectonic plates) are driven by geothermal 
energy— heat that is produced beneath Earth's surface. It is thought that most of this heat results 
from the decay of radioactive isotopes that were already present when Earth was formed. L(The 
reader completely unfamiliar with the terminology and physics of radioactivity may wish to look 
ahead to Box 8-3.) The most important isotopes in this respect are uranium-238 (half-life 4.5 
billion yr), thorium-232 (half-life 14 billion yr), and potassium-40 (half-life 1.3 billion yr). 
Notwithstanding the rather low concentration of these isotopes in Earth's crust, the energy 
released by their continuing radioactive decay is enough to account approximately for the 
observed rate of heat flow to the surface. The very long half-lives of these isotopes guarantee, 
moreover, that this source of energy for geological change will have been diminished only 
slightly a billion years hence. 

The processes of melting and resolidification, sinking and uplifting, the motion of tectonic plates 
of continental scale, the gouging and pushing of massive glaciers, and the different rates of 
weathering and erosion associated with different climates and different combinations of exposed 
rocks have combined to produce a tremendous variety of geological features. _The importance 
of these features to human beings is severalfold. The landforms— plains, mountains, valleys, and 
so on— are one major determinant of the extent to which different parts of the planet's surface are 
habitable. The soils that have resulted from geological and biological processes over 


It is possible that there is some additional contribution by frictional heat generation resulting 
from tidal forces on the molten and plastic parts of Earth's interior. 

The reader interested in pursuing this complex but fascinating subject should consult one of 
the several good geology books listed at the end of this chapter. 

the millennia are another (and more limiting) Acterminant of how many people can be supported 
and where. The zones of earthquakes and volcanism present serious environmental hazards to 
humans. And the distribution of fossil fuels and metals in scattered deposits far richer than the 
average crustal abundance is a geological phenomenon of enormous practical importance. 

Although the processes that produced these features often act imperceptibly slowly in human 
terms, the temptation to consider the geological forces to be beyond human influence— or to take 
for granted their contributions to human well-being— must be resisted. Soil that has taken a 
thousand years to accumulate can be washed or blown away in a day through human 
carelessness; and there is evidence that the activities of human societies worldwide— cultivation, 
overgrazing, deforestation, construction— have doubled the prehistoric rate of sediment transport 

to the sea (Chapter 6 and Chapter 11). Earthquakes are widely feared and are called natural 
disasters, but the lack of foresight in planning and construction that has characterized the 
development of human settlements in active earthquake zones suggests that the consequences are 
due as much to human ignorance and irresponsibility as to nature's harshness. There are 
circumstances, moreover, in which human activities actually cause earthquakes (injection of 
liquid wastes into rock formations, for example), and conceivably there may someday be 
technological means by which the frequency of strong earthquakes can be diminished. Without 
concentrated deposits of mineral ores, industrial civilization as we know it could not have arisen; 
they represent a coincidental natural subsidy for society, provided by the work of natural energy 
flows over eons. The notion that technological civilization is now clever enough to do without 
this subsidy, once it is used up, by extracting needed materials from common rock is a dubious 
one. (We will examine this idea more closely in Chapter 9.) 


Most forms of life on Earth require the simultaneous availability of mineral nutrients, certain 
gases, and water in liquid form. The boundaries of the biosphere—which are fuzzy, rather than 
sharp—can be defined as the places where the concentration of one or more of these essentials 
drops too low to sustain life. The principal reservoirs of available mineral nutrients are soil and 
sediment, the main reservoir of the needed gases is the atmosphere, and the primary supply of 
water is, of course, in the oceans. Where they meet, these reservoirs intermingle to produce the 
most fertile parts of the biosphere: the upper layers of soil, where gases and moisture readily 
penetrate, and the shallower parts of the oceans, where nutrients from the land and the bottom 
mingle with dissolved gases and light that penetrates downward from the surface. 

The oceans include not only some of the planet's most hospitable environments for life (and, 
almost surely, the environment where life began) but also make up by far the largest single 
habitat on Earth's surface. They cover almost 71 percent of the planet and their volume is an 
almost incomprehensible 1.37 billion cubic kilometers (330 million mi ). The term hydrosphere 
refers not only to the oceans themselves, however, but also to the "extensions" of the oceans in 
other realms— the water vapor and water droplets in the atmosphere, the lakes and the rivers; the 
water in soil and in pockets deep in layers of rock; the water locked up in ice caps and glaciers. 
The sections that follow here examine, first, some of the important characteristics of the oceans, 
then the behavior of ice on Earth's surface, and, finally, the hydrologic cycle, which makes water 
so widely available even far from the seas. 

The Oceans 

More than 97 percent of the water on or near the surface of Earth is in the oceans ( Table 2-6 ) . 
This enormous reservoir is a brine (salts dissolved in water) of almost uniform composition. The 
concentration of the dissolved salts ranges from 3.45 percent (by weight) to about 3.65 percent, 
varying with depth and latitude. The density of seawater varies between 1.026 and 1.030 grams 
per cubic centimeter, depending on depth and salinity, compared to 1 .000 grams per cubic 
centimeter for fresh water at the reference temperature of 4° C (39° F). An average cubic meter - 
of seawater weighs 1027 kilograms 

9 0ne cubic meter (m 3 ) = 35.3 cubic feet = 264 gallons. 

TABLE 2-6 Water Storage in the Hydrosphere 


Average in stream channels 

Vapor and clouds in atmosphere 

Soil water (above water table) 

Saline lakes and inland seas 

Freshwater lakes 

Groundwater (half less than 800 m below Earth's surface) 

Ice caps and glaciers 


1 km = 264 billion gallons. 

Twenty percent of this total is in Lake Baikal in the Soviet Union. 
Source: Brian J. Skinner, Earth resources. 

(1000 km 3 / 





125 — 




TABLE 2-7 Composition of Seawater (excluding dissolved gases) (MT / km3) - 

Elements at more th 

mn 1000 MT/km 3 

Selected elements at less than 


H 2 




















































*1 MT= 1000 kg. 

Source: Edward Wenk, Jr., 'The physical 

resources of the ocean, p. 


(1.027 MT, or 1.13 short tons), of which about 36 kilograms is dissolved salts. Although most of 
this material is the familiar sodium chloride, more than half of all the known elements are 
present in seawater at trace concentrations or more. 

The concentrations of several elements in seawater are given in Table 2-1 . It is thought that this 
composition has remained essentially unchanged during most of geologic time. This would mean 
that the inflow of minerals reaching the oceans from rivers, from the atmosphere, and from 
undersea volcanoes has been roughly balanced by the outflow—namely, the incorporation of 
inorganic precipitates and dead organic matter into sediments on the ocean floor. _^_In a situation 
of equilibrium of this kind (with inflow balancing outflow it is easy to calculate the average time 
an atom of a given element spends in the ocean between entering it and leaving it. This is called 
the residence time; it is an important concept in the study of nutrient cycles and of pollution. The 
concepts of equilibrium and residence time, along with some related ideas that find widespread 
application in environmental sciences, are reviewed in Box 2-2. The residence times of 

10 A superb and detailed treatment of the processes maintaining the composition of the oceans 
appears in Ferren Maclntyre, "Why the sea is salt". This and other Scientific American articles 
on the oceans referred to in this section are collected in J. Robert Moore, ed., Oceanography. 

BOX 2-2 Flows, Stocks, and Equilibrium 

The terms mass balance, energy balance, input/output analysis, and balancing the books all refer 
to fundamentally the same kind of calculation—one that finds extensive application in physics, 
chemistry, biology, and economics and in the many disciplines where these sciences are put to 
use. The basic idea is very simple: everything has to go somewhere, and it is possible and useful 
to keep track of where and how fast it goes. 

The concepts and terminology are illustrated in the diagram here. A stock is a supply of 
something in a particular place— money in a savings account, water in a lake, a particular element 
in the ocean. The stocks can be measured in terms of value (dollars), volume (liters), mass 
(grams), energy (joules), number of molecules, or other units, but not time. Time appears, 
instead, in the complementary concept of flows, the inflow (or input) being the amount of 
commodity added to the stock per unit of time and the outflow (or output) being the amount of 
commodity removed from the stock per unit of time. Thus, flows are measured in units like 
dollars per year, liters per minute, grams per day, or joules per second (watts). In the diagram, 
the sizes of the flows are indicated by the widths of the arrows. In a savings account, the inflow 
is deposits plus interest, the outflow is withdrawals, and the stock is the balance at any given 

Clearly, if the inflow is greater than the outflow, the stock becomes larger as time passes; if 
outflow exceeds inflow, the stock shrinks. The change in the size of the stock in a given period is 
the difference between inflow and outflow, multiplied by the length of the period. (If the inflow 
and outflow vary during the period, one must use their averages.) In the event that the inflow and 
the outflow have exactly the same magnitude, the size of the stock remains constant. This last 
situation, where inflow and outflow balance is called equilibrium, or, more specifically, dynamic 
equilibrium (something is flowing, but nothing is changing). The more restrictive case where 
nothing at all is happening— that is, no inflow, no outflow— is called static equilibrium. 

In a state of equilibrium, there is not necessarily any relation between the size of the flows 
{throughput) and the size of the stock. _For example, a small lake in equilibrium may be fed by a 
large river and drained by an equally large one (small stock, large throughput), or a large 




lake in equilibrium may be fed by a small river and drained by an equally small one (large stock, 
small throughput). If one divides the size of a stock in equilibrium by the size of the throughput, 
one obtains a very useful quantity—the average residence time (t). In the example of the lakes, 
this is the average length of time a water molecule spends in the lake between entering and 
leaving. For a lake of 100 million cubic meters, fed and drained by two rivers with flows of 100 
cubic meters per second each, the average residence time would be given by: 

100,000,000 m- 
100 m 3 /sec 

= 1,000,000 sec 

which is about twelve days. A smaller volume 

and/or a larger throughput would produce a shorter residence time. 

The concept of residence time is useful not only for describing geophysical processes but also for 
analyzing economic and biological ones. Economic "residence times" include replacement 
periods for capital and labor. And, to cite an example from biology, if the stock is the world's 
human population, then the inflow is the rate at which people are born, the outflow is the rate at 
which people die, and the average residence time is the life expectancy. Clearly, a given 
population size could be maintained at equilibrium by conditions of high throughput (high birth 
rate, high death rate, short life expectancy) or by conditions of low throughput (low birth rate, 
low death rate, long life expectancy). This subject is taken up in more detail in later chapters. 

Throughput means just what you would think— "what flows through"~and in general has the 
magnitude of the smaller of the inflow and outflow in a given situation. In equilibrium, inflow 
and outflow and throughput are all the same number. 

TABLE 2-8 Residence Times of Some Constituents of Seawater 

Residence time 


(million years) 











Source: Strahler and Strahler, 


geoscience, p. 


some important constituents of seawater are listed in Table 2-8 . 

The absolute quantities of materials dissolved in a cubic kilometer of seawater are quite large— 
175 metric tons of lithium, 3 metric tons of copper, 200 kilograms of silver, and 4 kilograms of 
gold, to mention some elements commonly regarded as scarce. Multiplying such numbers by the 
total volume of the oceans gives very large numbers, indeed, and these staggering quantities, 
combined with the ready accessibility of the oceans, have stimulated much discussion of mining 
seawater for its riches. In mining, it is the concentration of the material that counts, however (a 
subject to which we return in Chapter 9). To get the three tons of copper in a cubic kilometer of 
seawater, for instance, this desired material must somehow be separated from the billion metric 
tons of other elements mixed up with it, and this is not easy. 

Probably a more important reason for looking into the details of the composition of the oceans is 
to evaluate the seriousness of various kinds of ocean pollution. If pollutants such as lead and 
mercury, for example, are added to the oceans in sufficient quantities to alter substantially the 
natural concentrations of those elements over large areas, one might suspect that significant 
biochemical consequences could result. If, on the other hand, the discharge of an element into the 
ocean produces concentration changes small compared to natural variations in space and time of 
the concentration of this substance, little or no harm would be expected. 

One cannot assume, of course, that substances added to the oceans are quickly diluted by this 
vast volume of water. How far? How deep? How fast? are the questions, and the answers are 
found in the rather complicated patterns of horizontal circulation and vertical mixing in the 
oceans, as well as in the functioning of biological systems that may concentrate them (Chapter 
4). Vertical mixing is rapid only near the surface. The turbulence (violent mixing motions) 
produced by wave action at the surface penetrates only to a depth of from 100 to 200 meters, and 
this defines the thickness of the layer within which most of the absorbed solar energy is 

Below this warm, well-stirred surface layer is a transitional region called the thermocline, where 
the temperature drops rapidly. In this region there is usually less vertical motion than in the 
surface layer. Here heat penetrates partly by conduction (molecules passing on energy by jostling 
their neighbors) but mostly by convection (transport of energy by bulk motion of a warm 
medium) in large, slow eddies. There are two reasons for the relative lack of vertical motion in 
the thermocline: (1) motions originating at the surface have been damped out by friction before 
penetrating so deep and (2) the colder water near the bottom of the thermocline tends to be 
denser than the warmer water near the top of this layer, stifling thermal circulation. ^_In some 
circumstances, however, variations in salinity can influence density enough to produce a vertical 
circulation, despite the countervailing influence of the temperature profile. The bottom of the 
thermocline lies between 1000 and 1500 meters below the surface; from this level down, the 
temperature is nearly uniform and lies in the range from 0° to 5° C. (Seawater freezes at -2° C, or 
about 28.4° F.) Like the thermocline, this deepest ocean layer is thermally stratified, with the 
coldest water lying at the bottom. In the deep layer, moreover, salinity increases with depth; the 
saltier water is, the denser it is, so the salinity profile and the temperature profile both place the 
densest water at the bottom, inhibiting vertical mixing. 

This simplified view of the vertical layering of the oceans is illustrated schematically in Figure 
2-5 . Note that the stratification breaks down near the poles, where 

Thermal circulation, or thermal convection, occurs when warm fluid (liquid or gas), finding 
itself below colder, denser fluids, rises, while the colder material sinks. This is what happens 
when a fluid, such as water in a pan, is heated from below. The ocean is heated mainly at the 



H I 




20 j : 

( C) <V try •*#*) 

i* 30 Wtuttal 

FIGURE 2-5 Schematic diagram of the vertical structure of oceans. 

the cold layer extends all the way to the surface. That the surface waters in the Arctic and 
Antarctic oceans should be considered outcroppings of the deep layer that extends throughout the 
world ocean is suggested not only by patterns of temperature and salinity but also by the 
distribution of certain creatures. The huge Greenland shark, for example, once thought to inhabit 
only Arctic waters, has been photographed three or four kilometers deep in waters off Baja 
California and in the Indian ocean. — 

The stable stratification of the oceans also breaks down at scattered places and times far from the 
poles, as in upwellings in which winds push the surface water away from a steep continental 
slope and cold water rises from below to replace it (off the coast of Peru, for example) or when 
rapid cooling of surface water under unusual circumstances causes it to sink. The mean residence 
times for water in the various ocean layers illustrate the relative rarity in space and time of large 
vertical movements: a typical water molecule in the mixed layer may spend 10 years there, 
whereas one in the thermocline spends 500 years before reaching the deep layer, and a water 
molecule in the deep layer typically spends 2000 or 3000 years before reaching one of the upper 
layers. Clearly, it must be assumed that most substances added to the oceans near the surface and 
dissolved there will remain near the surface for years, being diluted only by the small fraction of 
the ocean water that makes up the well mixed top layer (between 3 and 5 percent). 

Horizontal circulation in the oceans is considerably faster than vertical mixing. Water in the 
main currents, which generally involve only the mixed layers, typically moves at speeds of 1 
kilometer per hour (km/hr), and occasionally up to 5 kilometers per hour. Thus, an object or a 
substance being carried in the current might easily move 1000 kilometers in a month and cross 
an ocean in, six months to a year. (The main oceanic surface currents appear on the map in 
Figure 2-6 .) The principal features are the circular movements, called gyres, centered in the 
subtropical latitudes (25° to 30° north and south of the equator), an equatorial countercurrent 
(most prominent in the Pacific) that provides a return pathway for water that would otherwise be 
piling up against Asia and Australia, and the Antarctic circumpolar current, flowing 
uninterrupted from west to east around the far southern part of the globe. These currents are 

1 9 

John D. Isaacs, The nature of oceanic life. 

orf fho ocntara 

January anHm* 
U*H on 30-yvar r««tl 
^ana> U S hfydrograpTK ONc 
Warm currants ► 

Cool eurponu _ 

Mercator's projection FIGURE 2-6 Main oceanic surface currents, 
by a complex interaction of the effects of the winds, Earth's rotation, and the placement of 

1 1, 

continents and islands. By moving enormous quantities of water-sometimes warm, sometimes 

cold—from one region to another, the ocean currents exert a major influence on climate, a subject 
taken up in more detail later. The two mightiest currents on Earth, the Antarctic circumpolar 
current and the main branch of the Gulf Stream, each carries some fifty times the combined flow 
of all the world's rivers. — 

The horizontal circulation in the deep layer of the ocean is much less thoroughly mapped than 
that of the surface layer and has been widely supposed to be much less vigorous. Typical speeds 
in these deep currents have been thought to be on the order of 0.1 kilometer per hour or less. An 
increasing number of direct measurements of deep ocean currents now suggest a much more 
vigorous deep-ocean circulation, however, involving powerful eddies 100 kilometers or more in 
horizontal extent, containing currents of 0.5 to 1 kilometer per hour. 15 One would expect the 
general flow to be from the poles toward the equator, inasmuch as some cold water enters the 
deep layer from above at the poles and some rises in upwellings closer to the equator. But the 
actual situation is made quite complicated by Earth's rotation, by the irregular distribution of 
landmasses, and by the complex topography of the ocean floor. 

It has been true historically and is still true today that the usefulness of the oceans to civilization 
and, in turn, civilization's impact on the oceans have been greatest in the shallower waters at the 
edges of the continents. This 

See, for example, R. W. Stewart, The atmosphere and the ocean. 
P. H. Kuenen, Realms of water, p. 47. 

F. Bretherton, Recent developments in dynamical oceanography. 

is so partly for simple reasons of accessibility, and partly because of the particular fertility of the 
near-shore waters and the richness of the underlying sediment in minerals of economic interest. 
The term continental shelf 'refers to that part of the near-shore underwater topography that is 
actually an extension of flatlands on a continent itself. Although the outer edge of a continental 
shelf is often defined as the line along which the depth of the water reaches 200 meters, a less 
arbitrary boundary is the point where there is a marked increase in the downward slope of the 
bottom. The steeply sloping region just beyond this boundary is called the continental slope. The 
"foothills" leading from the ocean floor to the seaward edge of the continental slope are called 
the continental rise. 

Using the definition of continental shelf just given, it has been estimated that continental shelves 
underlie 7.5 percent of the area of the oceans (an area equal, however, to 18 percent of Earth's 
land area). 16 These shelves vary in width from essentially nothing to 1500 kilometers, and their 
seaward edges vary in depth from 20 to 550 meters (the average depth at the edge is 133 meters). 
The circulation patterns in shallow, continental-shelf waters are complex, and the residence times 
of dissolved substances over the shelf can be surprisingly long — as much as several years to 
migrate from the coastline to the outer edge of a wide shelf like that off the east coast of the 
United States. 

Glaciers and Sea Ice 

Fifteen thousand years ago, much of what is now continental shelf was dry land. Sea level was 
130 meters lower than it is today. Where was the 45 million cubic kilometers of water (about 12 
billion billion gallons!) this difference in sea level represents? It was locked up in the great 
glaciers of the ice age. In the warmer period in which we find ourselves today, the water that 
remains frozen as ice still far exceeds all other reservoirs of fresh water on Earth ( Table 2-6 ). 
Were this ice to melt, sea level would rise another 80 meters. 

It is important in this connection to distinguish between glaciers and sea ice. A glacier is a sheet 
of ice formed on land when accumulated snow is compressed and hardened into ice by the 
weight of overlying layers. Sea ice is ice formed from seawater; it floats on the ocean's surface, 
although it may be attached to land at its edges. 

The glaciers that usually come to mind when one hears this term are the scattered "mountain and 
valley glaciers" that occur throughout the world's high mountain ranges — the Himalayas, Andes, 
Rockies, and Alps, for example. The larger glaciers of this variety are some tens of kilometers 
long, a kilometer or more across, and a few hundred meters thick. These glaciers are constantly 
in motion, being fed by snowfall at their surfaces in the higher elevations and moving downhill 
as the deep layers, under great pressure, flow as a plastic solid. (Perhaps the easiest way to 
visualize what is going on in such flow, which as we noted earlier also occurs in rock in Earth's 
mantle, is to consider the plastic solid to be an extremely viscous fluid.) The speed of advance 
varies along the length of the glacier, but is typically 100 meters per year or more in the main 

body of the larger mountain glaciers. The advance of such glaciers is terminated by melting of 
the tongue of the glacier at the lower end. 

By far the greatest part of the world's inventory of ice — more than 99 percent — is tied up in a 
second kind of glacier, the land ice or ice sheets, that cover the bulk of the Greenland and 
Antarctic landmasses. The formation of such a sheet requires an arctic climate, sufficient 
precipitation, and fairly flat land. The ice layer that results covers essentially the entire landscape 
in a gently sloping dome, interrupted only by a few projecting mountain peaks. The Greenland 
ice sheet covers an area of 1.74 million square kilometers (about 80 percent of the total area of 
Greenland) and has an average thickness of about 1600 meters (5250 ft). The Antarctic ice sheet 
covers 13 million square kilometers with ice up to 4000 meters thick (13,000 ft) and averaging 

perhaps 2300 meters. About 91 percent of the world's ice thus is in the Antarctic sheet and 

about 9 percent in the Greenland sheet. These ice sheets, like mountain and valley glaciers, 

K. O. Emery, The continental shelves. 
17 Press and Siever, Earth, p. 371; Strahler and Strahler, Environmental geoscience, pp. 434- 

are in motion, carrying to the sea the ice formed from precipitation in the central regions. Typical 
speeds are some tens of meters per year on the ice sheet proper, but they can be much higher — 
hundreds, and even thousands, of meters per year — where certain glacial tongues meet the sea. 

Where the ice sheets meet the sea in broad expanses, they may extend into the ocean as more-or- 
less floating ice shelves, from tens to hundreds of meters thick. In the Antarctic, these shelves 
reach widths of hundreds of kilometers. The largest, the Ross Ice Shelf, covers more than 
500,000 square kilometers. Icebergs originate when great masses of ice break off from the tips of 
glacial tongues or the edges of ice shelves and are carried away (often into shipping lanes) by 
currents (see Figure 2-7 ). 

Sea ice, as distinguished from floating extensions or pieces of glaciers, is formed by the freezing 
of seawater on the ocean surface. The North Pole ice pack, with a mean extent of about 10 
million square kilometers, is a collection of slabs of sea ice floating on the Arctic Ocean. In 
winter, these slabs are frozen together and attached to land at various points around the ocean's 
periphery. In summer, some of the slabs break apart and are separated by narrow strips of open 
water, and the southern limit of the ice retreats northward. The sea ice, which begins to form at — 
2° C, is porous, and the enclosed cavities often contain water saltier than seawater. Glacial ice, 
by contrast, consists of fresh water, being simply compacted and recrystallized snow. 

The maximum thickness of sea ice is only between 3 and 5 meters. Once it reaches this 
thickness, the layer of ice insulates the underlying water so well that no more can freeze — heat is 
supplied from the deeper water faster than the surface layer can lose it through the ice. (Ice is a 
poor conductor of heat and snow an even poorer one, which is why snow igloos stay so warm 
inside.) If the average thickness of the North Pole ice pack is 2 meters, it contains less than one- 

1 Q 

hundredth as much ice as the Greenland ice sheet. Of course, melting of the sea ice would have 

no direct effect on sea level, even if the volume of this ice were much greater; the ice is floating, 
thus displacing an amount of water equal to its weight, so 

FIGURE 2-7 A glacier feeding ice into the sea in Paradise Bay, Antarctica. No large icebergs are 

visible in this picture. (Photo by P. R. Ehrlich.) 

Ten million km of area multiplied by 0.002 km average thickness is 20,000 km of ice in the 
polar pack, compared to 1.7 million km multiplied by 2.2 km average thickness, or 2,700,000 
km of ice, in the Greenland sheet. 

it is already contributing exactly as much to the level of the oceans as it would if it melted. Aside 
from the sea-level issue, however, which relates solely to glacial ice sheets, the sea ice has great 
importance for climate. 

The Hydrologic Cycle 

Although oceans and ice caps contain some 99.3 percent of all the water on Earth ( Table 2-6 ), 
the fraction of 1 percent residing at any given time in the atmosphere, in lakes and streams, and 
in soil and subsurface layers plays unique and important roles. The flow of water on the surface 
is a major determinant of the configuration of the physical environment. Soil moisture is 
essential to most terrestrial plant life. The stocks and flows of ground and surface water are 

major links in the transport and cycling of chemical nutrients and important determinants of what 
kinds and intensities of human activity can be supported in what locations. And water in the 
atmosphere has several functions that are central to shaping climates. 

The set of processes that maintain the flow of water through the terrestrial and atmospheric 
branches of the hydrosphere is called the hydrologic cycle. The cycle includes all three physical 
states of water ~ liquid, solid (ice and snow), and gas (water vapor). It also includes all of the 
possible transformations among these states — vaporization, or evaporation (liquid to gas); 
condensation (gas to liquid), freezing (liquid to solid); melting, ox fusion (solid to liquid); and 
sublimation (gas to solid, or the reverse). 

The principal flows in the hydrologic cycle are: (1) evaporation of water from the surface of the 
oceans and other bodies of water, and from the soil; (2) transpiration of water by plants, the 
result of which is the same as that of evaporation — namely, the addition of water vapor to the 
atmosphere; (3) horizontal transport of atmospheric water from one place to another, either as 
vapor or as the liquid water droplets and ice crystals in clouds; (4) precipitation, in which 
atmospheric water vapor condenses (and perhaps freezes) or sublimates and falls on the oceans 
and the continents as rain, sleet, hail, or snow; and (5) runoff, in which water that has fallen on 
the continents as precipitation finds its way, flowing on and under the surface, back to the 
oceans. Because it is difficult and not particularly useful to distinguish between the contributions 
of evaporation and transpiration on the continents, these two terms are often lumped together as 
evapotranspiration . 

The magnitudes of these flows, averaged over all the continents and oceans and expressed in 
thousands of cubic kilometers of water per year, are shown in Figure 2-8 . ^These magnitudes 
are based on the assumption that the various components of the hydrosphere are in equilibrium, 
which is at least a good first approximation. That is, on a year-round average, inflows and 
outflows for the atmosphere, the oceans, and the continents all balance. (For example, in 
thousands of cubic kilometers, the atmosphere receives 62 + 456 = 518 as evaporation from the 
surface and gives up 108 + 410 = 518 as precipitation.) 

The magnitude of the flows in the hydrologic cycle is more readily grasped if one thinks of the 
flows in terms of the equivalent depth of water, averaged over the surface area involved. In these 
terms, the world's oceans annually lose to evaporation a layer of water 1.26 meters deep (about 4 
feet) over their entire surfaces, gaining back 1.14 meters from precipitation and 0.12 meters from 
the discharge of rivers and groundwater. The continents receive precipitation each year 
equivalent to a layer of water 0.73 meters (29 in) deep over their entire surface areas, of which 
0.42 meters is lost to evaporation and 0.31 meters makes up the runoff. 

Combining the foregoing information on equilibrium flows with the information on stocks in the 
hydrosphere summarized in Table 2-6 permits us to estimate the average residence time of water 
in the different parts of the cycle (see Box 2-2). These residence times, which are of great 
importance in analyzing the transport of pollutants, as well as nutrients, by the hydrologic cycle, 
are listed in Table 2-9 . There is an enormous range, from the average nine days a water molecule 
spends in the atmosphere between being evaporated from the surface and falling again as 
precipitation, to the 10,000 years a 


The values presented here are at the high end of a range of published estimates, in which the 
differences of professional opinion amount to as much as 25 percent. 

£ O 



fyp owa n ^ w iw 

FIGURE 2-8 The hydrologic cycle (1000 km3/yr). (Data from M. I. Budyko, 1974.) 
TABLE 2-9 Residence Times of Water Molecules in the Hydrologic Cycle 

Location time 

Atmosphere 9 days 

Rivers (typical speed, 1 m/sec) 2 weeks 

Soil moisture 2 weeks to 1 year 

Largest lakes 1 years 

Shallow groundwater (speed, 1-10 m/day) 10s to 100s of years 

Mixed layer of oceans (1st 150 m) 120 years 

World ocean 3000 years 

Deep groundwater up to 10,000 years 

Antarctic ice cap 10,000 years 

Sources: Computed from Table 2-6 and Figure 2-8 or adapted from SMIC, 
Inadvertent climate modification, and Strahler and Strahler, Environmental 

molecule of water typically spends as ice between falling in a snowflake on the Antarctic ice 
sheet and rejoining the ocean with the melting of an iceberg. It is also important to remember 
that there are large deviations from the average in any given category — a water molecule may 
fall in a raindrop not nine days but an hour after being evaporated from Earth's surface; another 
may wander not two weeks but two years in the delta of the Amazon River before reaching the 

sea. Nevertheless, the average residence times can provide useful insights into a variety of 
important problems, and the approach can be refined whenever information more pertinent than 
global averages is available. 

The balance between precipitation and evapotranspiration varies widely from continent to 
continent, as shown in Table 2-10 . The size of the runoff (the difference between precipitation 
and evapotranspiration) is a measure of how much water is potentially available for domestic and 
industrial uses by society (including dilution and removal of wastes) and for the other functions 
that flowing water performs. Note in Table 


TABLE 2-10 Average Water Balance of the Continents 





1 (km /yr) 





















North America 





South America 





Includes entire USSR. 

Source: Budyko, 

M. I., 

p. 227 

2-10 the remarkable fact that South America has a runoff per unit of surface area almost three 
times that of North America, the continent with the next greatest runoff. It is perhaps not so 
surprising, then, that the discharge of the Amazon River, which drains the wettest third of South 
America, amounts to about a seventh of the runoff of the entire world. 

Much of the runoff on the continents takes place not on the surface but beneath it. Although the 
quantities can only be estimated, it is clear that most rivers receive at least as much of their flow 
from see page through the ground as from flow over the ground; and a certain amount of water 
reaches the oceans via flowing aquifers and see page at the edges of the continents without ever 

joining a surface river at all. Water beneath the land's surface is called soil moisture, or soil 

water, when it is distributed in the first meter or so of soil (a zone defined by the depth of 
penetration of the roots of most plants.) Below the zone of soil moisture is an intermediate zone 
where the water percolates downward through open pores in the soil and rock; and below this is 
the water table, marking the surface of the body of groundwater that saturates the soil or rock in 
which it finds itself, filling all pores and spaces in the soil or rock completely. The groundwater 
extends downward until it is limited by an impermeable layer of rock. In some circumstances, 
there are successive layers of groundwater {aquifers) separated by impermeable layers of rock. 
The absolute lowest limit of groundwater is probably about 16 kilometers from the surface, 
where the pressure is so great that all pores are closed and any rock becomes impermeable. 

Most groundwater is flowing, albeit very slowly [10 meters per day (m/d) in coarse gravel near 
the surface, more commonly 1 m/d, and much more slowly at greater depths.] At 1 meter per 
day, of course, it takes almost three years to move 1 kilometer. Figure 2-9 is a schematic diagram 
of the zones and flows of subsurface water, showing the intersection of the water table and a 
surface river. 


or lake 

FIGURE 2-9 Zones of subsurface water. (After Ackerman, Colman, and Ogrosky, in A. N. 
Strahler and A. H. Strahler, Environmental geoscience.) 


See, for example, Strahler and Strahler, Environmental geoscience, Chapter 12, and Kuenen, 
Realms of Water, Chapter 5. 

The energy that drives the hydrologic cycle is energy from the sun ~ indeed, this function is the 
largest single user of the solar energy reaching Earth's surface. The reason so much energy is 
required is that it takes a great deal of energy to evaporate water ~ 2250 joules per gram at the 
boiling point of 100° C and 2440 joules per gram at Earth's average surface temperature of 15° 
C. (This is the highest heat of vaporization of any known substance.) It takes fifty times as much 
energy to evaporate a gram of water as it does to lift it to an altitude of 5 kilometers. The energy 
used to evaporate the water is stored as latent heat of vaporization (see Box 2-1), which is 
released to the environment as heat whenever and wherever the water vapor condenses into 
liquid. Thus, energy delivered by the sun at one point on Earth's surface may be released high in 

the atmosphere over a point 1000 kilometers away. This mechanism of redistributing energy by 
the transport and condensation of water vapor is a major determinant of Earth's climate. 

As noted above, the energy the sun supplies at the time of evaporation reappears as heat at the 
time of condensation. Similarly, the smaller amount of solar energy that does the work of lifting 
the water vapor against the force of gravity appears as frictional heat when falling droplets of 
condensed vapor collide with molecules of air and when rushing mountain streams rub against 
their rocky beds. That all the energy the sun supplies to terrestrial processes comes back again in 
one form or another is not coincidence or quirk, but an illustration of the first law of 
thermodynamics — the law of conservation of energy. Further excursions into the machinery of 
the physical world — and of human technology — will require some familiarity with this law and 
with its companion, the second law of thermodynamics, so an introduction to both is provided in 
Box 2-3. 


The blanket of gases that makes up Earth's atmosphere has many functions. Of the four elements 
required in greatest quantity by living organisms [carbon (C), oxygen (O), hydrogen (H), 
nitrogen (N)] the atmosphere provides the main reservoir of one (N), the most accessible 
reservoir of two others (C, O), and an essential link in the continuous recycling of the fourth (H, 
in the form of H20). The atmosphere is substantial enough to protect the organisms on Earth's 
surface from a variety of harmful particles and radiations that reach the planet from the sun and 
from space, but it is transparent enough to permit an adequate amount of life-giving sunlight to 
penetrate to that surface. Acting as a thermal insulator, the atmosphere keeps Earth's surface 
much warmer, on the average, than it would be if there were no atmosphere. And the stirrings of 
the atmosphere, transporting energy and moisture from one place to another, are a major part of 
the patterns of climate so important to the character and distribution of life. 

For simplicity, we begin our investigation of the atmosphere by ignoring its internal vertical and 
horizontal motions and considering its properties as a static body of gas. 


The term air refers to the particular mixture of gaseous compounds making up the atmosphere. 
The average composition of this mixture, not including water, is shown in Table 2-11 . An 
important property of gases is that a given number of molecules at a given temperature and 
pressure will occupy almost exactly the same volume, regardless of the mass or size of the 
molecules. This property has led to the use of the somewhat confusing terms percent by volume 
or fraction by volume to describe the relative abundance of the various constituents of gaseous 
mixtures. That is, if three-quarters of the molecules in a container of fixed volume are gas A and 
one-quarter are gas B, one can think of gas A as "occupying" three-fourths of the volume and gas 
B as "occupying" one-fourth. (What is happening in reality, of course, is that both gases, mixed 
together, occupy the whole volume, with gas A accounting for three-fourths of the pressure in 
the volume and gas B accounting for one-fourth of the pressure.) We will use the term molecular 
fraction (number of molecules of a constituent divided by the total number of molecules in the 

mixture), because it is unambiguous and works for solids and liquids as well as gases. The reader 
should simply be aware that this term is interchangeable with the term 


TABLE 2-11 Average Composition of Clean Dry Air 





Carbon Dioxide 





Nitrous Oxide 




N 2 



CO 2 



CH 4 


N 2 

H 2 














fraction of air 




320 ppm 

18 ppm 

5.2 ppm 

2.9 ppm 

1.1 ppm 

0.5 ppm 

0.5 ppm 

0.01 ppm 

Mass fraction 
of air 




486 ppm 

12 ppm 

0.7 ppm 

1.6 ppm 

3.2 ppm 

0.8 ppm 

0.03 ppm 

0.02 ppm 

Source: Garrels, Mackenzie, and Hunt, Chemical cycles. 

BOX 2-3 Availability, Entropy, and the Laws of Thermodynamics 

Many processes in nature and in technology involve the transformation of energy from one form 
into others. For example, light from the sun is transformed, upon striking a meadow, into thermal 
energy in the warmed soil, rocks, and plants; into latent heat of vaporization as water evaporates 
from the soil and through the surface of the plants; and into chemical energy captured in the 
plants by photosynthesis. Some of the thermal energy, in turn, is transformed into infrared 
electromagnetic radiation heading skyward. The imposing science of thermodynamics is just the 
set of principles governing the bookkeeping by which one keeps track of energy as it moves 
through such transformations. A grasp of these principles of bookkeeping is essential to an 
understanding of many problems in environmental sciences and energy technology. 

The essence of the accounting is embodied in two concepts known as the first and second laws of 
thermodynamics. No exception to either one has ever been observed. The first law, also known 
as the law of conservation of energy, says that energy can neither be created nor destroyed. If 
energy in one form or one place disappears, the same amount must show up in another form or 
another place. In other words, although transformations can alter the distribution of amounts of 
energy among its different forms, the total amount of energy, when all forms are taken into 
account, remains the same. The term energy consumption, therefore, is a misnomer; energy is 
used, but it is not really consumed. One can speak of fuel consumption, because fuel, as such, 
does get used up. But when we burn gasoline, the amounts of energy that appear as mechanical 
energy, thermal energy, electromagnetic radiation, and other forms are exactly equal all together 
to the amount of chemical potential energy that disappears. The accounts must always balance; 
apparent exceptions have invariably turned out to stem from measurement errors or from 

overlooking categories. The immediate relevance of the first law for human affairs is often stated 
succinctly as, "You can't get something for nothing." 

Yet, if energy is stored work, it might seem that the first law is also saying, "You can't lose!" (by 
saying that the total amount of stored work in all forms never changes). If the amount of stored 
work never diminishes, how can we become worse off? One obvious answer is that we can 
become worse off if energy flows to places where we can no longer get at it — for example, 
infrared radiation escaping from Earth into space. Then the stored work is no longer accessible to 
us, although it still exists. A far more fundamental point, however, is that different kinds of 
stored work are not equally convertible into useful, applied work. We can therefore become 
worse off if energy is transformed from a more convertible form to a less convertible one, even 
though no energy is destroyed and even if the energy has not moved to an inaccessible place. The 
degree of convertibility of energy — stored work — into applied work is often called availability. 

Energy in forms having high availability (that is, in which a relatively large fraction of the 


BOX 2-3 {Continued) 

stored work can be converted into applied work) is often called high-grade energy. 
Correspondingly, energy of which only a small fraction can be converted to applied work is 
called low-grade energy, and energy that moves from the former category to the latter is said to 
have been degraded. Electricity and the chemical energy stored in gasoline are examples of high- 
grade energy; the infrared radiation from a light bulb and the thermal energy in an automobile 
exhaust are corresponding examples of lower-grade energy. The quantitative measure of the 
availability of thermal energy is temperature. More specifically, the larger the temperature 
difference between a substance and its environment, the more convertible into applied work is 
the thermal energy the substance contains; in other words, the greater the temperature difference, 
the greater the availability. A small pan of water boiling at 100° C in surroundings that are at 20° 
C represents considerable available energy because of the temperature difference; the water in a 
swimming pool at the same 20° C temperature as the surroundings contains far more total 
thermal energy than the water in the pan, but the availability of the thermal energy in the 
swimming pool is zero, because there is no temperature difference between it and its 
surroundings. With this background, one can state succinctly the subtle and overwhelmingly 
important message of the second law of thermodynamics: all physical processes, natural and 
technological, proceed in such a way that the availability of the energy involved decreases. 
(Idealized processes can be constructed theoretically in which the availability of the energy 
involved stays constant, rather than decreasing, but in all real processes there is some decrease. 
The second law says that an increase is not possible, even in an ideal process.) As with the first 
law, apparent violations of the second law often stem from leaving something out of the 
accounting. In many processes, for example, the availability of energy in some part of the 
affected system increases, but the decrease of availability elsewhere in the system is always large 
enough to result in a net decrease in availability of energy overall. What is consumed when we 
use energy, then, is not energy itself but its availability for doing useful work.The statement of 
the second law given above is deceptively simple; whole books have been written about 

equivalent formulations of the law and about its implications. Among the most important of 
these formulations and implications are the following: 

In any transformation of energy, some of the energy is degraded. 

No process is possible whose sole result is the conversion of a given quantity of heat 

(thermal energy) into an equal amount of useful work. 

No process is possible whose sole result is the flow of heat from a colder body to a hotter 


The availability of a given quantity of energy can only be used once; that is, the property of 

convertibility into useful work cannot be "recycled." 

In spontaneous processes, concentrations (of anything) tend to disperse, structure tends to 

disappear, order becomes disorder. 

That Statements 1 through 4 are equivalent to or follow from our original formulation is readily 
verified. To see that statement 5 is related to the other statements, however, requires establishing 
a formal connection between order and availability of energy. This connection has been 
established in thermodynamics through the concept of entropy, a well defined measure of 
disorder that can be shown to be a measure of unavailability of energy, as well. A statement of 
the second law that contains or is equivalent to all the others is: all physical processes proceed in 
such a way that the entropy of the universe increases. (Not only can't we win — we can't break 
even, and we can't get out of the game!) 

Consider some everyday examples of various aspects of the second law. If a partitioned 
container is filled with hot water on one side and cold water on the other and is left to itself, the 
hot water cools and the cold water warms ~ heat flows from hotter to colder. Note that the 
opposite process (the hot water getting hotter and the cold getting colder) does not violate the 
first law, conservation of energy. That it does not occur illustrates the second law. Indeed, many 
processes can be imagined that satisfy the first law but violate the second and therefore are not 
expected to occur. As another example, consider adding a drop of dye to a glass of water. 
Intuition and the second law dictate that the dye will spread, eventually coloring all the water ~ 
concentrations disperse, order (the dye/no dye ar- 


BOX 2-3 {Continued) 

rangement) disappears. The opposite process, the spontaneous concentration of dispersed dye, is 

consistent with conservation of energy but not with the second law. 

A more complicated situation is that of the refrigerator, a device that certainly causes heat to 
flow from cold objects (the contents of the refrigerator — say, beer — which are made colder) to a 
hot one (the room, which the refrigerator makes warmer). But this heat flow is not the sole result 
of the operation of the refrigerator: energy must be supplied to the refrigeration cycle from an 
external source, and this energy is converted to heat and discharged to the room, along with the 
heat removed from the interior of the refrigerator. Overall, availability of energy has decreased, 
and entropy has increased. 

One illustration of the power of the laws of thermodynamics is that in many situations they can 
be used to predict the maximum efficiency that could be achieved by a perfect machine, without 
specifying any details of the machine! (Efficiency may be defined, in this situation, as the ratio 
of useful work to total energy flow.) Thus, one can specify, for example, what minimum amount 
of energy is necessary to separate salt from seawater, to separate metals from their ores, and to 
separate pollutants from auto exhaust without knowing any details about future inventions that 
might be devised for these purposes. Similarly, if one is told the temperature of a source of 
thermal energy — say, the hot rock deep in Earth's crust — one can calculate rather easily the 
maximum efficiency with which this thermal energy can be converted to applied work, 
regardless of the cleverness of future inventors. In other words, there are some fixed limits to 
technological innovation, placed there by fundamental laws of nature. (The question of how far 
from the maximum attainable efficiencies industrial societies operate today is taken up in 
Chapter 8.) 

More generally, the laws of thermodynamics explain why we need a continual input of energy to 
maintain ourselves, why we must eat much more than a pound of food in order to gain a pound 
of weight, and why the total energy flow through plants will always be much greater than that 
through plant-eaters, which in turn will always be much greater than that through flesheaters. 
They also make it clear that all the energy used on the face of the Earth, whether of solar or 
nuclear origin, will ultimately be degraded to heat. Here the laws catch us both coming and 
going, for they put limits on the efficiency with which we can manipulate this heat. Hence, they 
pose the danger (discussed further in Chapter 11) that human society may make this planet 
uncomfortably warm with degraded energy long before it runs out of high-grade energy to 

Occasionally it is suggested erroneously that the process of biological evolution represents a 
violation of the second law of thermodynamics. After all, the development of complicated living 
organisms from primordial chemical precursors, and the growing structure and complexity of the 
biosphere over the eons, do appear to be the sort of spontaneous increases in order excluded by 
the second law. The catch is that Earth is not an isolated system; the process of evolution has 
been powered by the sun, and the decrease in entropy on Earth represented by the growing 
structure of the biosphere is more than counterbalanced by the increase in the entropy of the sun. 
(The process of evolution is discussed in more detail in Chapter 4.) 

It is often asked whether a revolutionary development in physics, such as Einstein's theory of 
relativity, might not open the way to circumvention of the laws of thermodynamics. Perhaps it 
would be imprudent to declare that in no distant corner of the universe or hithertounexplored 
compartment of subatomic matter will any exception ever turn up, even though our intrepid 
astrophysicists and particle physicists have not yet found a single one. But to wait for the laws of 
thermodynamics to be overturned as descriptions of everyday experiences on this planet is, 
literally, to wait for the day when beer refrigerates itself in hot weather and squashed cats on the 
freeway spontaneously reassemble themselves and trot away. 


fraction by volume, often used elsewhere for gases. In many applications it is also useful to work 
with the mass fraction (grams of constituent/gram of mixture). This is a more precise statement 

? 1 

of what is meant by the common terra fraction by weight or percent by weight. — 

A mole of any substance is 6.02 x 10 molecules, and the mass of a mole is equal to the 
molecular weight of the substance in grams. For example, the mass of a mole of nitrogen gas (N 
2) is 28 grams. It is often convenient to speak of air as if it were a single substance; the term a 
mole of air means 6.02 x 10 molecules, of which 78.09 percent are nitrogen molecules, 20.95 
percent are oxygen molecules, and so on. Such a collection of molecules has a mass of about 29 
grams, which is called the molecular weight of air. (These definitions will be of importance later 
in interpreting what is meant by pollution standards expressed in different ways.) 

Although nitrogen and oxygen comprise 99 percent of dry air, the trace constituents carbon 
dioxide (CO 2 ) and ozone (O 3 ) play exceedingly important roles because of the special 
properties of these molecules, as described below. Methane, nitrous oxide, and hydrogen also 
have roles in atmospheric chemistry and physics, albeit smaller ones. Argon, helium, krypton, 
and neon, by contrast, are chemically inert, monatomic gases, whose presence in the atmosphere 
is of interest only as resources for certain applications in technology. — 

Water Vapor 

The water content of the atmosphere varies greatly from place to place and time to time. Three 
commonly used measures of water content are absolute humidity, specific humidity, and relative 
humidity. Absolute humidity is the mass of water vapor per unit volume of air, and it varies from 
almost zero over the driest deserts to around 25 grams per cubic meter over jungles and tropical 
seas. Specific humidity is the mass of water vapor per unit mass of air. (A closely related term, 
the mixing ratio, is the mass of water vapor mixed with each unit mass of dry air.) Relative 
humidity, usually expressed as a percentage, is the ratio of the actual molecular fraction of water 
vapor in air to the molecular fraction corresponding to saturation at the prevailing temperature. 
{Saturation refers to the condition that ensues if air is left for a long time in a sealed container 
partly filled with pure water; the number of molecules of water vapor per unit volume of air 

under these circumstances depends only on the temperature.) Relative humidity usually is 

between and 100 percent, but under special circumstances (supersaturation) it can significantly 
exceed 100 percent. 

Under ordinary circumstances, water vapor in the atmosphere begins to condense into droplets of 
liquid, forming clouds, as soon as the relative humidity exceeds 100 percent by even a small 
amount. The process of condensation is greatly facilitated by the virtually universal presence in 
the atmosphere of small particles that provide surfaces where the condensation can commence. 
Called condensation nuclei when they perform this function, these particles include salt crystals 
formed by the evaporation of sea spray, dust raised by the wind, ash from volcanoes and forest 
fires, decomposed organic matter, and, of course, particles produced by various technological 
activities. Even in "unpolluted" air, particles that might serve as condensation nuclei are 
seemingly abundant in absolute terms (more than 100 particles per cubic centimeter), but the 
extent of condensation and precipitation apparently are related to specific physical characteristics 
of the condensation nuclei as well as to their number. 

The molecular fraction of water vapor corresponding to saturation increases as temperature 
increases — warm air can "hold" more water vapor than cool air. Accordingly, there are two ways 
in which relative humidity can be raised from less than 100 percent to more, initiating 

9 1 

Weight means the force exerted upon a mass by gravity. Weight and mass are more or less 
interchangeable (using the relation, weight equals mass multiplied by acceleration of gravity) 
only if one stays on Earth's surface, where gravity is nearly constant. An astronaut has the 
same mass on the surface of the moon as on Earth, but a very different weight, because the 
acceleration of gravity on the moon is much less than on Earth. 


For discussions of how the atmosphere came to have the composition it does, the reader 
should consult Preston Cloud and Aharon Gibor, The oxygen cycle; and Preston Cloud, ed., 
Adventures in earth history. 

The often encountered definition of saturation as "the maximum amount of water vapor air 
can hold at a given temperature" is not quite correct. A good discussion of this and the 
following points is given by Morris Neiburger, James G. Edinger, William D. Bonner, 
Understanding our atmospheric environment, Chapter 8. 

condensation and perhaps precipitation: (1) more water vapor can be added to the air by 
evaporation from an exposed water surface; (2) the air can be cooled so that the vapor content 
corresponding to saturation falls. At a given vapor content (a fixed specific humidity), the 
temperature at which the relative humidity reaches 100 percent is called the dew point. Addition 
of water vapor to the air by evaporation is a slow process, but cooling of the air can be very 
rapid. Rapid cooling to below the dew point is the mechanism immediately responsible for most 
condensation phenomena — the appearance of dew and fog at night as air is cooled by radiation 
of heat to the night sky; formation of clouds and rain in updrafts as the air is cooled by 
expansion; and formation of beads of water on the outside of a pitcher of ice water on a hot day, 
as air adjacent to the pitcher is cooled by contact with the cold surface. 

Pressure, Temperature, and Vertical Structure 

The pressure exerted by the atmosphere on objects at Earth's surface is essentially equal to the 
weight of the overlying air, which at sea level amounts on the average to 10.3 metric tons per 
square meter (14.7 lb/in ). This amount of pressure, defined as I atmosphere, is the same as 
would be exerted at sea level by a column of water about 10 meters high (33 ft) or a column of 
mercury 760 millimeters (mm) high (29.92 in). This means that the mass of the atmosphere is 
only equivalent to that of a 10-meter layer of water covering Earth (you can check this in Table 
2-2 ) and that pressure under water increases by the equivalent of I atmosphere for every 10 
meters of depth. The usual metric unit for the measurement of atmospheric pressure is the 
millibar; 1 millibar is 100 newtons (N) per square meter (see Box 2-1 for the definition of the 
newton and I atmosphere is 1013.25 millibars. 

Atmospheric pressure is not ordinarily perceived as a force because it acts equally in all 
directions (up, down, sideways); organisms are not crushed by it because the gases and liquids in 
tissue are also at atmospheric pressure, so the inward and outward forces balance. Pressure 

becomes perceptible as a (painful) pressure difference if the pressure outside an organism 
changes more rapidly than the interior pressure can accommodate (an example would be the pain 
in one's ears associated with a rapid change in altitude). 

Unlike water, whose density at the bottom of the deepest ocean trenches at pressures of hundreds 
of atmospheres is only a few percentage points higher than its density at the surface, the air in the 
atmosphere is highly compressible — that is, density increases markedly as pressure increases. 
Indeed, air behaves very much like a "perfect gas," for which pressure (p), density (p), and 
temperature (7) are related by the equation/) = pRT For this equation to be valid, the temperature 
T must be measured with respect to absolute zero, the temperature at which there is no molecular 
motion. Temperature measured from this zero point, which is the same for all substances, is 
called absolute temperature, and the corresponding unit of measurement in the metric system is 
the degree kelvin. 24 The R in the equation is the gas constant, which for dry air equals 287 joules 
per kilogram per degree kelvin. According to the perfect gas equation, the density of dry air 
varies in direct proportion to pressure, if temperature is held constant. 

Because the atmosphere is compressible, its mass is concentrated in the lower layers. Forty 
percent of the air in the atmosphere lies below the altitude of the summit of Mount Whitney in 
California's Sierra Nevada range (4.4 km) and two-thirds lies below the altitude of the summit of 
Mount Everest (8.9 km). The density of air at an altitude of 12 kilometers, where most subsonic 
jet airliners fly, is about one-fifth the density at sea level. The average variation of pressure and 
temperature with altitude above sea level is shown in Figure 2-10 . 

The atmosphere is subdivided into horizontal layers according to the pattern of temperature 
variation. The lowest layer, called the troposphere, is characterized by a rather uniform average 
rate of temperature decline with altitude of 6.4° C per kilometer. Almost all the atmo- 


Absolute zero, or degrees kelvin (K) equals -273.15° C. An attempt was made recently to 
standardize the unit of absolute temperature as simply the kelvin, rather than the degree 
kelvin, but the change has not been generally adopted. 


PRESSURE [millibars) 
:o 400 600 

4Q - 


g 20 



i r 





- r - 













FIGURE 2-10 Variation of atmospheric temperature and pressure with altitude (idealized). 

spheric phenomena that govern climate take place in the troposphere. The top of the troposphere 
is called the tropopause, where the temperature decline stops and a layer of uniform temperature 
at about -55° C commences. The tropopause is typically found at an altitude of from 10 to 12 
kilometers, but it ranges from a low of 5 or 6 kilometers at the poles to around 18 kilometers at 
the equator. 

The stratosphere extends from the tropopause up to the stratopause (about 50 km) and is 
characterized over much of this interval by temperatures increasing with altitude (reaching 
almost 0° C at the stratopause). The gaseous composition of the stratosphere is essentially the 
same as that at sea level, with two significant exceptions. First, there is very little water vapor in 
the stratosphere; the mixing ratio is typically two or three parts per million (ppm), or 1000 to 

10,000 times less than is common near sea level. Second, there is a great deal more ozone in the 
stratosphere than in the troposphere; the maximum molecular fraction of ozone is 10 ppm near 
25 km 


altitude, or 1000 times more than the average for the whole atmosphere. The air pressure and 

density at the top of the stratopause are on the order of a thousandth of the values at sea level. 

Above the stratosphere lies the mesosphere (to about 90 km), wherein the temperature again 
decreases with altitude. The composition of the mesosphere remain much like that of the lower 
layers, except for certain trace constituents such as water vapor and ozone. The troposphere, 
stratosphere, and mesosphere together are called the homosphere, referring to their relatively 
uniform composition. Above the mesosphere is the thermosphere (temperature again rising with 
altitude), which contains the heterosphere (so named because the molecular constituents are 
there separated into distinct layers of differing composition) and the ionosphere (referring to the 
presence of free electrons and the positively charged ions from which the electrons have been 
stripped). The thermosphere has no well defined upper limit; its density at 100 kilometers is 
around one-millionth of atmospheric density at sea level, and by 10,000 kilometers it has faded 
off to the density prevailing in interplanetary space. 

Radiant Energy Flow in the Atmosphere 

What accounts for the complicated way in which temperature changes with altitude in the 
atmosphere? The answer involves the way in which different atmospheric constituents interact 
with radiant energy arriving from the sun and with radiant energy trying to escape Earth into 
space. The same processes, of course, determine how much and what kinds of energy reach 
Earth's surface, so they are crucial in determining the conditions that govern life. To understand 
these processes requires at least a modest acquaintance with the character of radiant energy (or 
electromagnetic radiation), and this is supplied in Box 2-4. 

The energy in the electromagnetic radiation reaching the top of Earth's atmosphere from the sun 
is distributed over a range of wavelengths, as shown in Figure 2-11 . One can determine from 
such a graph that about 9 

22 - 

21 - 


0.19 - 
0.16 - 






017- 'I 






Black body spectral 
irradiance curve 
(temperature - 6000*K) 



. Solar spectral 
ir radiance curve 


I- I 


02 - 



I I I I 1 

02 04 

14 16 18 2 

6 8 10 12 

FIGURE 2-11 Solar irradiance spectrum and 6000°K blackbody radiation reduced to mean solar 
distance. (From Neiburger, Edinger, and Bonner, 1973.) 

percent of the total incoming energy is in the ultraviolet part of the electromagnetic spectrum, 41 
percent is in the visible part of the spectrum, and 50 percent is in the infrared part. A significant 
part of this energy is prevented from reaching Earth's surface by gaseous constituents of the 
atmosphere that are opaque to certain wavelengths. This opacity is due not to reflection but to 
absorption (the energy in the radiation is absorbed by the gas molecules, warming the 
atmospheric layers where these processes take place). 

The depletion of incoming radiation by absorption in atmospheric gases is summarized in Table 
2-12 . The main results are that ultraviolet solar radiation with wavelengths less than 0.3 microns 
(u) is almost completely absorbed high in the atmosphere, and infrared solar radiation is 
substantially depleted through absorp- 


Study of Critical Environmental Problems (SCEP), Man's impact on the global environment, 
p. 41. 


Box 2-4 Electromagnetic Radiation 

Light, X-rays, radio waves, infrared radiation, and radar waves are all variations of the same 
thing — phenomena with the interchangeable names electromagnetic radiation, electromagnetic 
waves, and radiant energy. 

Energy in this form travels at the speed of light (c, or 299,792 km/sec in vacuum), and — as the 
words in vacuum imply — requires no material medium to support the energy flow. This property 
contrasts with the other, much slower forms of energy transport (conduction and convection) 
which do require a medium. Convection involves the bulk motion of matter; conduction involves 
molecular motion. What moves in the case of electromagnetic radiation is a combination of 
electric and magnetic fields of force. 

For many purposes, it is useful to visualize this pattern as a traveling wave, as shown in the 
diagram here. There the curve denotes the spatial pattern of the strength of the electric or 
magnetic field, and the pattern is moving to the right with speed c. (Actually, there should be 
separate curves for the electric and magnetic fields, but this detail need not trouble us here.) The 
wavelength (X) is the distance between successive crests or troughs. _At any fixed point along 
the path of the wave, the field is seen to oscillate with frequency (v) related to wavelength and 
the speed of light by the relation v = c/L 




» K * 

The different forms of electromagnetic energy are distinguished by their different wavelengths 
(or, equivalently, their frequencies, since one can be computed from the other using the relation 
Xv8, = c). Visible electromagnetic radiation has wavelengths between 0.40 microns (violet light) 
and 0.71 microns (red light). A micron (ju) is one-millionth of a meter. The entire range of 
wavelengths that have been observed, from tiny fractions of a micron to tens of kilometers, is 
called the electromagnetic spectrum. Some types of electromagnetic radiation occupying 
different parts of the spectrum are indicated in the table below. 

Type of radiation 


radar (microwaves) 


Wavelength range 

1-10 m 

1-30 cm 
0.71-100 n 

Type of radiation 

Wavelength range 




0.40-0.71 (x 
0.10-0.40 (x 
10" 5 -10" 2 n 

The way electromagnetic radiation interacts with matter depends in a complicated manner on 

Unfortunately, there are more concepts in science than there are Greek letters. Thus, for 
example, lambda (X) represents wavelength in physics and finite rate of increase in population 
biology (Chapter 4). 

the wavelength of the radiation, on its intensity (energy flow per unit of area, measured — say — 
in watts per square meter and on the properties of the matter. Radiant energy that encounters 
matter may be transmitted, reflected, or absorbed. To the extent that radiation of a given 
wavelength is transmitted, the material is said to be transparent to that wavelength; to the extent 
that the radiation is reflected or absorbed, the material is said to be opaque to that wavelength. 
Most materials are transparent to some wavelengths and opaque to others. Many gases, for 
example, are rather opaque to most ultraviolet wavelengths but transparent to radio waves, 
visible light, and X-rays. Human flesh is opaque to visible light but transparent to X-rays. 

Often, transmission, reflection, and absorption all take place at once. Consider a glass window in 
the morning sunlight: the glint off the window indicates that reflection is taking place; the room 
behind the window is illuminated and warmed, so there is certainly transmission; and the 
window itself gets warm, so absorption is happening, too. 

Both transmission and reflection can be direct or diffuse. Direct means that a beam of 
electromagnetic radiation arriving from a single specific direction is sent on or sent back in a 
single specific direction. Diffuse means that the incoming beam is split up and sent on or sent 
back in many different directions. The phenomenon that produces diffuse transmission and 
reflection is called scattering. All light reaching the ground on a completely overcast day is 
diffuse light that has been scattered by the water droplets in the cloud layer. 

All matter that is warmer than absolute zero can emit radiation. (The "machinery" by which 
emission occurs has to do with the behavior of the electrons in matter. We will not dwell on this 
machinery here, summarizing instead only the main results of its operation.) Some substances 
emit only radiation of certain wavelengths, and all substances can absorb only the wavelengths 
they can emit. A body that absorbs all the radiation that hits it in all wavelengths is called a 
blackbody, a useful idealization that is approached in the real world but never quite reached. A 
blackbody is not only a perfect absorber, but also a perfect emitter; the amount of 
electromagnetic energy emitted at any given wavelength by a blackbody of a specified 
temperature is the theoretical maximum that any real body of that temperature can emit at that 
wavelength. The total amount of radiation emitted by a blackbody is proportional to the fourth 
power of the body's absolute temperature. The wavelength at which a blackbody emits most 

intensely decreases in inverse proportion to the absolute temperature — that is, the higher the 
temperature, the shorter the wavelength. 

The characteristics of the sun as a source of electromagnetic radiation are very closely 
approximated by those of a blackbody with a temperature of 5800° K. The wavelength at which 
its emission is most intense is 0.5 microns, corresponding to blue-green light (see Figure 2-11 ). 


TABLE 2-12 

Absorption of Solar Radiation by Atmospheric Gases 
Wavelength range 

(ju) Fate of radiation 


Less than 0.12 All absorbed by O 2 and N 2 above 100 km 

0.12-0.18 All absorbed by O 2 above 50 km 

0.18-0.30 All absorbed by O 3 between 25 and 50 km 

0.30-0.34 Part absorbed by O 3 

0.34-0.40 Transmitted to Earth almost undiminished 

0.40-0.71 Transmitted to Earth almost undiminished 

0.71-3 Absorbed by CO 2 and H 2 O, mostly below 1 km 
Source: Neiburger, Edinger, and Bonner, Understanding our atmospheric environment. 

tion by carbon dioxide and (especially) by water vapor at lower altitudes. The atmosphere's gases 
are almost completely transparent to the visible wavelengths, where the intensity of solar 
radiation reaches its peak ( Figure 2-11 ) , and to the "near-ultraviolet" radiation, with 
wavelengths just shorter than the visible. (This nearultraviolet radiation is an important 
contributor to sunburn.) Significantly, the only atmospheric gas that is opaque to ultraviolet 
radiation between 0.18 and 0.30 microns in wavelength is ozone. Without the trace of ozone that 
exists in the stratosphere, this radiation would reach Earth's surface ~ where it could be 
extremely disruptive to the life forms that evolved in the absence of these wavelengths (Chapter 
1 1). The absorption of ultraviolet radiation by ozone in the stratosphere has a second important 
effect — it produces the stratospheric heating that causes temperature to increase with altitude in 
this layer of the atmosphere, with consequences discussed in the next section. 

Some of the incoming radiation that is not absorbed by atmospheric gases is scattered by them 
(Box 2-4). Some of the scattered radiation returns to space (diffuse reflection), and some reaches 
the ground as diffuse solar radiation. The physics of scattering by the molecules in air is such 
that blue light is scattered much more than is red light. The sky appears blue because what 
reaches the eye is mostly scattered light and hence — owing to the preferential scattering of blue 
wavelengths by air molecules ~ mostly blue. That the sun appears red at sunset is precisely the 

same phenomenon at work; then one is looking at direct-beam radiation — the unscattered part - 
from which the blue has been removed (by scattering) and in which mainly the red remains. 

Absorption and scattering of solar radiation are done by aerosols as well as by gases. An aerosol 
is a suspension of solid or liquid particles in a gas. Fog, clouds, smoke, dust, volcanic ash, and 
suspended sea salts are all aerosols. Whether a given aerosol acts mainly as an absorber or 
mainly as a scatterer depends on the size and composition of the particles, on their altitude, and 
on the relative humidity of the air they are in. Certainly, the aerosols that interact most strongly 
with solar radiation are clouds, which at any given time cover about half Earth's surface with a 
highly reflective layer. 

The reflectivity of a surface or a substance is called its albedo (formally, albedo equals reflected 
energy divided by incoming energy). This property depends not only on the characteristics of the 
surface, but also on the angle at which the incoming radiation strikes the surface (the angle of 
incidence). The albedo is much higher at shallow angles of incidence than at steep (nearly 
perpendicular) angles. The albedo of clouds ranges from 0.25 (thin clouds, perpendicular 
incidence) to more than 0.90. 

Averaged around the year and around the globe, the amount of energy that penetrates to Earth's 
surface is about half of what strikes the top of the atmosphere. Of that which penetrates, 
somewhat more than half is diffuse and somewhat less than half is direct. Upon 


TABLE 2-13 Albedo of Various Surfaces 

Surface Albedo 

Snow 0.50-0.90 

Water 0.03-0.80 

Sand 0.20-0.30 

Grass 0.20-0.25 

Soil 0.15-0.25 

Forest 0.05-0.25 

Sources: SMIC, Inadvertent 
climate modification; 
Neiburger, Edinger, and 
Bonner, Understanding our 
atmospheric environment. 

reaching the surface, this energy meets several fates. Some is reflected, with the albedo varying 
from land to water and from one type of vegetation to another ~ and, of course, depending on the 
angle of incidence ( Table 2-13 ) . Some is absorbed by the melting or sublimation of ice or snow, 
or the vaporization of water. Some is absorbed to warm the surface and objects on or under it. 
And a tiny fraction is captured and transformed into chemical energy by the process of 
photosynthesis in plants. 

The fate of solar energy striking the top of Earth's atmosphere is summarized in Figure 2-12 . 
Note that the average albedo of the Earth-atmosphere system is about 0.28, of which two-thirds 
is accounted for by clouds. 

Now from the first law of thermodynamics (Box 2-3), which says energy can neither be created 
nor destroyed, and from elementary considerations of stocks and flows (Box 2-2), one can draw 
some important conclusions about energy flow in the Earth-atmosphere system. First, the rate at 
which Earth's atmosphere and surface are absorbing solar energy must be matched by the rate at 
which the system loses energy, or else the amount of energy stored on the surface and in the 
atmosphere would be steadily changing. In other words, if the system is to be in equilibrium, 
outflow must equal inflow. Lack of equilibrium, if it occurred, would mean a changing stock of 
energy in the Earth-atmosphere system. This could manifest itself as an upward or downward 
trend in mean temperature (a changing stock of thermal energy), or in mean absolute humidity or 
mean volume of ice and snow 





























FIGURE 2-12 The fate of incoming solar radiation. Figures represent global annual averages. 


(a changing stock of latent energy of vaporization and fusion), or in mean quantity of organic 
matter (a changing stock of chemical energy). Of course, different parts of Earth's surface and 
atmosphere at different times of year are out of equilibrium — temperature, humidity, snow 
cover, and quantity of vegetation generally change dramatically with seasons. Inflow and 
outflow of energy at a given time and place in general do not balance. But they must balance on a 
year-round average for the whole globe or else there will be year-to-year changes in the mean 
values of these indices of stored energy. Such changes have occurred in the past as cooling 
periods leading into ice ages and warming periods leading out of them. These changes and the 
potential for further ones, with or without human influence, are discussed below. As a first 
approximation, however, it is reasonable to assume that inflows and outflows of energy are in 
balance on the global, annual average. 

The second conclusion is that the same considerations of inflow, outflow, and equilibrium that 
apply to the Earth-atmosphere system as a whole must apply separately to its components. If the 
atmosphere is to be in equilibrium in a global, time-averaged sense, it must be losing energy at 
the same rate it is receiving it. The same must hold for Earth's surface. 

Clearly, then, the energy flow shown in Figure 2-12 is not the complete picture. The figure 
shows a net accumulation of energy in the atmosphere (gas molecules, clouds, dust) equal to 22 
percent of the solar energy that strikes the top of the atmosphere and a net accumulation of 
energy at the surface equal to 47 percent of total incoming solar energy. 

What is the fate of this absorbed energy, amounting in all to 69 percent of the solar input? The 
answer is this: after running the machinery of winds, waves, ocean currents, the hydrologic 
cycle, and photosynthesis, the energy is sent back to space as terrestrial radiation. 

Terrestrial radiation refers to the electromagnetic radiation emitted by Earth's surface and 
atmosphere in accordance with the principles summarized in Box 2-4: the amount of energy 
radiated per unit of area is proportional to the fourth power of the absolute temperature of the 
radiating substance, and the wavelength of most intense radiation is inversely proportional to the 
absolute temperature. If the Earth-atmosphere system were a blackbody at temperature, T, the 
exact relation for the rate of emission of radiation, S, would be: 
S=oJ 4 . 

If S is measured in watts per square meter and T is measured in degrees kelvin, then the 
proportionality constant a (the Stefan-Boltzmann constant) has the numerical value 5.67 X 10- . 
On the assumption that the energy returned to space as terrestrial radiation should exactly 
balance the 69 percent of incoming solar energy that is absorbed, one can compute the effective 
blackbody temperature of the Earth-atmosphere system to be about 255° K (- 18° C or about 0° 
F). This is the temperature a perfect radiator would have to have in order to radiate away into 
space the same amount of energy per unit of area as the Earth-atmosphere system actually does 
radiate away, on the average. 

Actually, the Earth-atmosphere system closely resembles a blackbody radiator, so the above 
temperature is not unrealistic. Why, then, is that temperature so much lower than the observed 
mean surface temperature of 288° K (15° C, or 59° F)? The reason is that most of the terrestrial 

radiation actually escaping from the Earthatmosphere system is emitted by the atmosphere, not 
the surface. (You can see from Figure 2-10 that the temperature 255° K corresponds to an 
altitude of about 5 kilometers, or halfway between sea level and the tropopause.) Most of the 
terrestrial radiation emitted by the warmer surface does not escape directly to space because the 
atmosphere is largely opaque to radiation of these wavelengths. The wavelength of radiated 
energy, you should recall, increases as the temperature of the radiator decreases: Earth, being 
much cooler than the sun, emits its radiation at longer wavelengths. The peak intensity of 
terrestrial radiation occurs at a wavelength of around 10 microns, which is in the infrared part of 
the spectrum, in contrast to solar radiation's peak intensity at a wavelength of about 0.5 microns 
in the visible part of the spectrum. — 

26 There is almost no overlap in the wavelength ranges of solar and terrestrial radiation; at 3 
microns, the intensity of solar radiation has fallen to 5 percent of its value at the 0.5-micron 
peak, while that of terrestrial radiation has only attained about 1 percent of its value at the 
10-micron peak. Meteorologists often refer to solar radiation as short-wave radiation and 
terrestrial radiation as long-wave radiation. 

The opaqueness of the atmosphere to outgoing infrared radiation from the surface is due mainly 
to three atmospheric constituents: carbon dioxide, water vapor, and clouds. (A more modest 
contribution is made by ozone.) Carbon dioxide absorbs infrared radiation in a narrow band of 
wavelengths around 3 microns, in another narrow band near 4 microns, and in the wavelengths 
between 12 and 18 microns. Water vapor absorbs infrared radiation in narrow bands around 1, 
1.5, and 2 microns, and in broader ones from 2.5 to 3.5 microns, from 5 to 8 microns, and from 
15 microns through the remainder of the infrared. Clouds are very much like blackbodies in the 
entire infrared part of the electromagnetic spectrum — they absorb most of the infrared radiation 
that reaches them. The infrared radiation absorbed by carbon dioxide, water vapor, and clouds is 
subsequently reradiated by these substances — much of it being sent back in the direction of 
Earth's surface and some escaping into space. The atmosphere, therefore, through the properties 
of clouds, water vapor, and carbon dioxide, acts as a thermal blanket that keeps Earth's surface 
about 33° C warmer than it would be without these constituents. 

Because of this effect of clouds, all else being equal, clear nights are colder than cloudy nights; 
in the absence of clouds, more infrared radiation leaving the surface escapes directly to space 
without being intercepted. The additional thermal-blanket effect of water vapor and carbon 
dioxide is sometimes called the greenhouse effect. This is because glass, like carbon dioxide and 
water vapor, is relatively transparent to visible radiation but more opaque to infrared radiation. 
Light enters a greenhouse more readily than heat can escape, a situation resembling that in the 
atmosphere. The main reason a greenhouse is warmer inside than outside, however, is that the 
glass prevents convection from carrying away sensible heat and latent heat of vaporization. — 
Thus, the term greenhouse effect is not entirely appropriate for the role of carbon dioxide and 
water vapor in the atmosphere. 

The flows of terrestrial radiation are summarized in Figure 2-13 , expressed as a percentage of 
solar energy striking the top of the atmosphere and computed as a global annual average. Note 

that the rate at which infrared radiation is emitted from Earth's surface actually exceeds the rate 
of solar input at the top of the atmosphere and is far larger than the rate at which solar energy is 
directly received at the surface. This large radiation output and the high temperature responsible 






















Net lou from curiae* - 1 14 • 24 ♦ 5 96-47 

FIGURE 2-13 Flows of terrestrial radiation. Percentage of solar energy incident at the top of the 

atmosphere (global annual averages). 


This fact has been demonstrated by building a greenhouse of rock salt (which is as transparent 
to infrared radiation as to visible radiation) next to a greenhouse of glass. The rock-salt 
greenhouse, simply by preventing convection, stayed almost as warm inside. (See, for 
example, Neiburger et al., Understanding.) 

for it are made possible by the large flow of infrared radiation reradiated to the surface by the 
atmosphere, as discussed earlier. Any alteration of the composition of the atmosphere by 
pollution can have an important influence on this downward reradiated energy flow (see Chapter 

Two important energy flows other than radiation are represented in Figure 2-13 b y broken lines. 
The larger of these flows is the transfer of energy from the surface to the atmosphere as latent 
heat of vaporization. That is, energy that has been used to evaporate water at the surface moves 
into the atmosphere in the form of the latent heat of vaporization that is associated with the water 
vapor (see Box 2-1). This energy is eventually surrendered to the atmosphere as sensible heat 
when the water vapor condenses. The magnitude of this surfaceto-atmosphere energy flow is 
equal to almost a quarter of the solar energy flow reaching the top of the atmosphere. The second 

major nonradiative flow (perhaps a fifth as large as that of latent heat) is the transfer of sensible 
heat from the surface to the atmosphere by conduction ~ that is, the warming of the air by 
contact with the surface. Sensible heat transported across the interface of surface and air in this 
way then moves upward in the atmosphere by convection. (Of course, in some places and at 
some times the atmosphere is warmer than the surface, with the result that sensible heat flows 
from the atmosphere to the surface rather than the reverse. Remember, we have been discussing 
the global, annual-average situation here.) 

Atmospheric Energy Balance and Vertical Motions 

With the information in the preceding section, one can construct average energy balances for 
Earth's surface, for the atmosphere, and for the Earth-atmosphere system as a whole. Such a set 
of balances is given in Table 2-14 , both in terms of the percentage of solar energy flow at the top 
of the atmosphere and in watts per square meter of Earth's surface area. Energy flow per unit of 
area is called flux. The solar flux through a surface perpendicular to the sun's rays at the top of 
the atmosphere is called the solar constant and is equal to about 1360 watts per square meter 
[1.95 calories (cal)/cm /minute (min), or 1.95 langleys/min, in units often used by 


meteorologists]. As a sphere (to a very good approximation), Earth's total surface area is 4 

times the area of the cross section it presents to the sun's rays. Hence, the average amount of 
solar energy reaching the top of the atmosphere per square meter of Earth's surface is just one- 
fourth of the solar constant, or 340 watts per square meter, as indicated in Table 2-14 . The 
average solar flux reaching Earth's surface is 173 watts per square meter of horizontal surface. 
Solar flux measured this way at the surface is often termed insolation. 

The difference between the amount of energy incident on Earth's surface as radiation (solar and 
terrestrial) and the amount leaving as radiation is called by meteorologists the net radiation 
balance, or sometimes just net radiation. This amount of energy flow, which as shown in Table 
2-14 averages 105 watts per square meter flowing into the surface for the whole globe, is of 
special meteorological significance because it is the amount of energy available for the 
climatically crucial processes of evaporation of water and surface-to-air transfer of sensible heat. 
Many discussions of human impact on climate use the net radiation balance, rather than the 
incident solar energy, as the yardstick against which civilization's disturbances are measured 
(Chapter 11). 

The pattern of Earth-atmosphere energy flows that has been described here provides the 
explanation for the observed temperature distribution in the lower atmosphere. Basically, the 
atmosphere is heated from the bottom — by direct contact with warm land or water surfaces, by 
the release of latent heat of vaporization when water vapor condenses (virtually entirely in the 
troposphere and mostly in its lowest third), and by the absorption of terrestrial infrared radiation 
in water vapor, clouds, and carbon dioxide. Water vapor and clouds are most abundant in the 
lower troposphere, so that is where a large part of the absorption takes place. These effects, 
differentially heating the atmosphere near the bottom, produce the general trend of decreasing 
temperature with altitude in the troposphere. Only above 

It is actually not known exactly how constant the solar constant really is (see, for example, S. 

Schneider and C. Mass, Volcanic dust, sunspots, and temperature trends). 

TABLE 2-14 Energy Balances for Earth's Surface, Atmosphere, and Surface-Atmosphere 
System (global annual averages, accurate to perhaps ± 10%) 

Percentage of solar energy Average energy flow 

flow at top of atmosphere 

Solar radiation reaching top of atmosphere 1 00 

(w/m ) 

Total inflow 1 00 

Solar radiation scattered from atmosphere 25 
Solar radiation scattered from surface 3 

Terrestrial direct radiation, surface to space 5 
Terrestrial radiation, atmosphere to space 67 

Total outflow 


Solar direct radiation 

Solar diffuse radiation, via clouds 

Solar diffuse radiation, via air, dust 

Terrestrial reradiation from clouds, vapor, 
CO 2 




Total inflow 


Reflected solar radiation 


Outgoing terrestrial radiation 


Latent heat to atmosphere 


Sensible heat to atmosphere 


Total outflow 


\ 1 JVlUor rllilvti 

Solar radiation interacting with clouds, 



Terrestrial radiation from surface, 



Latent heat from surface 


Sensible heat from surface 


Total inflow 
Solar radiation scattered to space 

















Percentage of solar energy Average energy flow 

(w/m ) 

flow at top of atmosphere 







Solar radiation scattered to surface 
Absorbed radiation reradiated to space 
Absorbed radiation reradiated to surface 

Total outflow 214 728 

These flows exceed 100 percent of incoming solar radiation because, in addition to the solar 
throughput, an internal stock 
of energy is being shifted back and forth between surface and atmosphere. 

Source: After Schneider and Dennett, Climatic barriers. 

the tropopause does atmospheric heating by absorption of ultraviolet solar radiation (particularly 
by ozone) reverse the trend and produce temperatures that increase with altitude (refer to Figure 
2-10 ). Ozone also absorbs outgoing terrestrial infrared radiation in a narrow wavelength band at 
9.6 microns, to which other atmospheric gases are largely transparent. 

The structure of the troposphere, with the warmest air n the bottom, promotes vertical instability 
(hot air rises, and cold air sinks). Sensible heat and latent heat of vaporization are thus 
transported upward by convection, and the troposphere tends to be vertically well mixed. The 
mixing time in the lower half of the troposphere -that is, the time it takes for a molecule at 5 













Inversions in the troposphere. Temperature versus altitude in different circumstances, 
altitude to change places with one at the surface — is usually on the order of a few days. (In 
convective storms it can be hours.) The stratosphere, by contrast, is vertically stratified; like the 
ocean, its warmest layer is on top, with progressively colder layers below, which tends to 
suppress vertical motion. The vertical mixing time in the stratosphere is on the order of a year or 
two.Under some circumstances, the usual temperature profile of the troposphere near the ground 
is altered so that temperature increases with altitude. In this situation (called an inversion), 
vertical mixing is suppressed over the altitude range where the temperature is increasing. 
(Vertical mixing is also suppressed in circumstances where temperature decreases with altitude 
but not rapidly enough to overcome the stratifying effect of the density variation.) Some of the 
main possibilities are indicated in Figure 2-14 . Inversions are of special importance in 
environmental science because they inhibit the dilution of pollutants, as is discussed further in 
Chapter 10. 

Variation of Incoming Solar Flux with Place and Season 

If the vertical energy flows discussed in the preceding sections were all there were to climate, it 
would not be so difficult to analyze. So far, however, we have only been working with energy 
flows averaged over the whole globe and the whole year; the real complexity lies in the tangled 
patterns of energy flow that arise from differences between day and night, summer and winter, 
land surfaces and water surfaces, and so on. Let us first consider the geometrical factors that 
produce variations from season to season and from latitude to latitude in the amount of solar 
radiation that strikes the top of the atmosphere. They are: 

Earth is essentially spherical. 

Earth travels around the sun in an orbit that is not quite circular. 

Earth spins on an axis that is tilted 23.5' from perpendicular to the plane of its orbit. 

Because Earth is a sphere, the maximum solar flux at any moment is received at the subsolar 
point (that is, the point that is directly "under" the sun, or equivalently, the point at which the sun 
appears to be directly overhead) and declining values of flux are received as one moves away 
from this point on the surface of the sphere. This is so because a flat surface of given area 
intercepts a maximum of the solar beam when the surface is perpendicular to the beam, and 
progressively less at angles away from the perpendicular. This situation is illustrated in Figure 2- 



Lttscri beam 

^—. * — i — - ■■ 
of wuoraf 

to tartan 

Of tWlfl nAta 

MflXfTMJni sfnoi^tf oft basin 
Of ftowortai 
partial to stffac* 
ol Earth ha™ 


Warn per 





FIGURE 2-15 Insolation at the top of the atmosphere. 

Earth's elliptical orbit around the sun deviates from a circle just enough to make the distance 
between Earth and sun vary by ± 1 .7 percent from the mean value of 149.6 million kilometers. 
This variation in distance produces a difference of ± 3.4 percent in solar flux (that is, the solar 
energy flow incident on a surface perpendicular to the sun's rays above the atmosphere varies 
from a maximum of 1406 w/m to a minimum of 13 14 w/m , the average of 1360 w/m being the 
"solar constant" given earlier). The minimum distance occurs in the first few days of January, 
when the Northern Hemisphere is having its winter and the Southern Hemisphere its summer, 
and the maximum occurs in the first few days of July. 

Clearly, the seasons are not produced by the slight ellipticity in Earth's orbit, but rather by the tilt 
of Earth's axis of rotation. The orientation of this tilt remains fixed as Earth circles the sun, as 
shown schematically in Figure 2-16 . This means that the Northern Hemisphere is tilted directly 

toward the sun at the June 21 solstice, corresponding to the first day of summer in the Northern 
Hemisphere and the first day of winter in the Southern Hemisphere, and the Southern 
Hemisphere is tilted directly toward the sun at the December 21 solstice. On June 21 the subsolar 

point is on the Tropic of Cancer (23.5 north latitude) and the entire area north of the Arctic 

Circle (66.5 north latitude) is illuminated by the sun during all twenty- four hours of the day. On 
December 21 the subsolar point is on the Tropic of Capricorn (23.5 south latitude), and the 
entire area within the Antarctic Circle is illuminated for all twenty- four hours of the day. At the 
equinoxes (March 21 and September 23) the subsolar point is on the equator, and day and night 
are each twelve hours long everywhere on Earth (except in the vicinity of the poles, which 
experience continuous twilight). 

The net effect of these geometrical aspects of the Earth-sun relationship is to produce strong 
north-south differences, or gradients, in the incoming solar flux perpendicular to Earth's surface 
at the top of the atmosphere. The size and shape of the gradients vary 


The subsolar point becomes a subsolar line as Earth performs its twenty- four hour rotation. 

FIGURE 2-16 Earth's orbit around the sun. 

with the seasons. Figure 2-17 shows the solar flux at the top of the atmosphere, in calories per 
square centimeter per day, as it varies throughout the year at three different latitudes in the 
Northern Hemisphere. Note the strong equator-to-pole contrast in the winter, which is reduced as 
summer approaches and actually reverses in midsummer. (The summer pole receives more 
energy per unit of surface area in midsummer than does the equator, because the sun is shining 
twenty-four hours a day at the summer pole.) 

The amount of solar radiation reflected back into space also varies with latitude (more is 
reflected at the extreme northerly and southerly latitudes, where the radiation strikes the surface 
at angles far from perpendicular) and with season (ice and snow reflect more than do vegetation 
and water). Finally, the amount of terrestrial radiation leaving the top of the atmosphere varies 
with latitude and season, because the emission of such radiation depends on temperature and 
other characteristics of the atmosphere and the surface below. An accounting of the 




1200 j- 

'; _ 

,..K - 

o 600 - 





-IOC - 

?QO - 

Jan Feb Mar Apr May June July Aug Sept. Oct Nov Dec 
FIGURE 2-17 Seasonal variation of solar flux on a horizontal surface outside the atmosphere. 

(From Gates, 1971.) 

radiation flows across an imaginary surface at the top of the atmosphere, then, must include 
incoming sunlight, outgoing reflected sunlight, and outgoing terrestrial radiation. Where the 
inflow exceeds the outflows across such a surface, it is said that the underlying column of 
atmosphere has a heating excess. Where the outflows exceed the inflow, there is said to be a 
heating deficit. 

The Machinery of Horizontal Energy Flows 

If there were no mechanisms to transfer energy horizontally over Earth's surface from regions of 
heating excess to regions of heating deficit, the result would be much greater extremes in 
conditions than actually exist. Energy would accumulate in the areas of heating excess until the 
additional outgoing infrared radiation produced by higher temperatures restored a balance; 
similarly, energy would be lost to space from areas of heating deficit until the drop in outgoing 
radiation associated with lower temperatures restored the incoming-outgoing balance in those 

areas. To say that the extremes of temperature on Earth's surface would be greater under these 
circumstances is an understatement. It is plain from Figure 2-17 , for example, that the region 
north of 80° north latitude receives no incoming radiation at all from mid-October to late 
February (although, of course, the emission of outgoing terrestrial radiation continues). In the 
absence of energy inflows from warmer latitudes, then, the radiative energy loss in winter at the 
latitudes having darkness twenty- four hours a day would cool the surface rapidly and 
continuously toward absolute zero. 

But there are horizontal energy flows — principally, the transport of sensible heat and latent heat 
of vaporization by the motions of the atmosphere and the transport of sensible heat by ocean 
currents. (These convective energy transfers are much bigger and faster than conduction. See 
Box 2-4, to review the difference.) These flows are driven largely by the north-south temperature 
differences arising from the radiation imbalances just described, and they are largely responsible 
for the general features of global climate. The overall pattern of the north-south energy flows on 
an annual average basis is indicated in Table 2-15 . 

The atmospheric motions that carry energy toward the poles as sensible heat and latent heat 
must, of course, be balanced by return flows of air toward the equator. Otherwise, air would be 
piling up at the poles ! Two kinds 

TABLE 2-15 Average Annual North-South Energy Flows (w/m2) 
Net radiation 
at top of atmosphere Net latent heat Net sensible Net sensible 

(incoming minus (precipitation heat transport heat transport 

outgoing) minus evaporation) by atmosphere by ocean 














Note: Sum of entries in any given zone is zero, denoting balance of inflows and outflows 

averaged for year. (Positive 

numbers represent net inflow; negative numbers represent net outflow.) 

Source: Modified from Budyko, in SMIC, Inadvertent climate modification, p. 91. 


Latitude zone 













0-1 0°N 







































of circulation that move heat poleward but keep the air distributed are particularly important. The 
first of these is the thermal circulation illustrated in Figure 2-1 8 A ; the poleward flow is high in 
the troposphere; the equatorward flow, near the surface. The second is the cyclonic circulation 
shown in Figure 2-1 8B , which can be thought of as taking place in a horizontal plane. Actually, 
Earth's major atmospheric circulations are generally combinations of vertical and horizontal 
components. The term 












Earth s surface 






FIGURE 2-18 North-south air flows with poleward transport of heat. A. Thermal circulation, 

cross-section looking west, parallel to Earth's surface in the Northern Hemisphere. B. Cyclonic 

circulation, looking down on Earth's surface in the Northern Hemisphere. 

wind is usually reserved for the horizontal motion, and the terms updraft and downdraft describe 
the vertical. The horizontal motion is almost always much faster than the vertical: typical wind 
speeds range from 1 to 20 meters per second; typical vertical speeds in large-scale atmospheric 

circulations are 100 times less, although local updrafts associated with clouds and mountain 
ranges may be 10 meters per second or more. Typically, then, a "parcel" of air (an arbitrarily 
defined collection of molecules whose behavior one chooses to trace) might move 100 
kilometers horizontally while rising one kilometer. 

Like solids, gases such as air move in response to the forces exerted upon them in ways 
described by Newton's laws of motion. One important kind of force in the atmosphere is 
associated with pressure gradients. The pressure-gradient force is a push from regions of high 
pressure, associated with high temperature and/or density, toward regions of low pressure. 
Another important force in the atmosphere is gravity, which exerts a downward pull on every 
molecule of air. (Vertical motions in the atmosphere generally are slow because the vertical 
pressure-gradient force usually balances the force of gravity almost exactly.) The distribution of 
regions of high pressure and low pressure in the atmosphere is complex, owing not only to the 
variations in insolation, reflection, and absorption with latitude and altitude, but also to local 
differences associated with topography and the distribution of vegetation, land, and water. But 
the atmospheric circulation patterns are not what would be expected from consideration only of 
the pressure gradients associated with these features, together with the force of gravity, because 
two other important factors come into play. These are the Coriolis deflection (usually 
inappropriately called the Coriolis force) and friction. 

The Coriolis deflection is a complication that arises from the rotation of the planet. Specifically, 
Earth tends to rotate out from under objects that are in motion over its surface (for example, fired 
artillery shells and moving parcels of atmosphere). That is, such objects do not go quite where 
they seem to be heading, because the place they were heading is rotating at a different velocity 
than the point of origin (rotational speed is highest at the equator, where a point must move some 
40,000 km/day, and lowest near the poles). Thus, in the Norther 


Hemisphere a parcel of air moving north will appear to a terrestrial observer to be deflected to 
the right (east), the direction of the Earth's rotation, because it will carry the higher velocity of its 
place of origin. The Coriolis "force" is the apparent extra force (besides pressure gradients, 
gravity, and any other "real" forces that may be acting) needed to explain to an observer on 
Earth's rotating surface the observed paths of moving objects. We say "apparent" because no 
work is done to produce the deflection — it is a function of the position of the observer. A person 
in space observing the trajectory of an artillery shell relative to the solar system (assume, for 
example, that the shell were visible but the rest of Earth were invisible) would not see any 
Coriolis deflection. The magnitude of the Coriolis deflection is greatest at the poles and zero on 
the equator; the magnitude also varies in direct proportion to the speed of an object ~ a 
stationary object is not subject to Coriolis deflection. ^The direction of the Coriolis deflection is 
always perpendicular to the direction of an object's motion. It changes the direction of motion 
but not the speed. Motions in the Northern Hemisphere are deflected to the right; motions in the 
Southern Hemisphere, to the left. 

Consider the effect of the Coriolis deflection in the Northern Hemisphere on the flow of air into 
a region of low atmospheric pressure from surrounding regions of higher pressure. The pressure- 

gradient force tries to drive the flow straight in, but the Coriolis deflection bends it to the right ( 
Figure 2-19 ). The resulting spiral patterns are actually visible in most satellite photographs of 
Earth, because the winds carry clouds along with them ( Figure 2-20 ). The spirals associated 
with low-pressure centers (clockwise in the Southern Hemisphere, counterclockwise in the 
northern) are called cyclones, and the outward spirals associated with high-pressure centers 
(clockwise in the Northern Hemisphere, counterclockwise in the southern) are called 

The force of friction adds two features to the wind patterns described thus far. It slows down the 
wind near Earth's surface — most dramatically in the first few tens of 




^ HIGH ^f 




FIGURE 2-19 Coriolis deflection of the wind. Light arrows denote the direction of pressure- 
gradient force, which would also be wind direction on a nonrotating Earth. Heavy arrows show 
the Coriolis deflection of the wind directly to the right (Northern Hemisphere). 


The magnitude of the Coriolis deflection associated with horizontal motion at velocity v m/sec 
at latitude (j) is 2Qv sinc|)/kg mass, where D. = 7.29 x 10-5 radians/sec is the angular velocity of 

Earth's rotation. (There are 2n radians, or 360 degrees, in one revolution.) See, for example, 
Neiburger et al., Understanding, pp. 99-104. 

FIGURE 2-20 Atmospheric circulation patterns as revealed by cloud distributions. (NASA). 

meters, but still significantly at altitudes up to a few hundred meters. And, because of the way 
the friction force interacts with the pressure-gradient force and the Coriolis deflection, it causes 
the direction of the wind to change with altitude for the first several hundred meters. The 
horizontal pressure gradients themselves may be quite different at one altitude than at another, 
which also gives rise to significant changes in the wind patterns as one moves away from Earth's 
surface. — 

The General Circulation 

The overall pattern of atmospheric motions resulting from the phenomena just described is called 
the general circulation. Its main features are illustrated in Figure 2-21 . The associated variation 
of average sea level pressure is indicated in Figure 2-22 . 

The surface circulation in the tropical regions north and south of the equator is dominated by the 
trade winds, blowing, respectively, from the northeast and the southeast. These very steady 
winds are associated with the pressure drop between the subtropical highs and the 


More detailed explanations of the operation of the friction force and the variation of pressure 
gradients with altitude are given in Neiburger et al., Understanding, pp. 109-1 14. 








W— w*Bteri«i 
PE— polar easterly 
STH— lublfopiCil higfis 
L— low 
H — high 

FIGURE 2-21 Main features of the general global circulation. Flows near the surface and cross- 
sections of the main circulations in the upper troposphere. 

9 q= N| _ Polar high 

Aleutian and 
Icelandic lows 


30° N 







zone (doldrums) 




Polar high 


FIGURE 2-22 Variation of average see-level pressure with latitude. (From Neiburger, Edinger, 

and Bonner, 1973). 

low-pressure doldrums on the equator. The trade winds are deflected from the direction of the 
pressure drop by the Coriolis force — to the right in the Northern Hemisphere, to the left in the 
southern — as explained above. The region where the trade winds meet to form a belt of easterly 
winds encircling the globe near the equator is called the intertropical convergence zone. 

The vertical part of the circulation in the tropics consists of thermal circulations of the form 
shown in Figure 2- 18 A , one immediately north of the equator and one immediately south. These 
are called Hadley cells, after the British meteorologist who first postulated their existence. In the 
Hadley cells, the air rising over the equator is moist as well as warm. As it rises, the air cools, 
whereupon some of the contained moisture condenses into droplets and falls as rain. The latent 
heat of 


vaporization released in this process helps drive the air farther upward, producing more 
condensation, more release of latent heat, and more rain. Thus, the air rising near the equator in 
the Hadley cells is largely "wrung out," producing in the process the very rainy climates for 
which the tropics are known. Having lost its moisture and some of its sensible heat in ascending, 
the air flowing poleward in the upper part of the Hadley cells continues to lose heat by radiating 
energy to space more rapidly than it absorbs radiant energy from the warmer atmospheric layers 
and the surface below. At around 30° north and south latitude, the relatively cold, dry air 

commences to sink. In sinking into higher pressure it is warmed by compression. This 
descending flow of warm, dry air in the 30* latitude belts is a major reason these belts are 
characterized by deserts all around the world. (The Sahara of northern Africa, the Kalahari of 
southern Africa, the Atacama of Chile, and the Sonoran desert of Mexico and the United States 
are examples.) Finally, as the dry air moves equatorward on the surface to complete its circuit, it 
picks up both heat and moisture from the increasingly warm surfaces of the land and water of the 

Thermal circulation patterns similar to the Hadley cells of the tropics are also found in the 
vicinity of the poles, but they are smaller and weaker than those on either side of the equator. 
The temperature and pressure differences driving the polar flow are less than those nearer the 
equator, and the transfers of moisture and latent heat are also much smaller. 

Although it was postulated at one time that there must be an indirect cell linking the equatorial 
and polar cells in each hemisphere — that is, a cell in which air sinks on the side toward the 
equator, flows poleward on the surface, and rises on the polar side — measurements indicate that 
this pattern is either extremely weak or entirely missing. Rather than being borne by such 
circulations, the poleward energy flow in the middle latitudes is accomplished instead by the 
great, swirling, horizontal flows associated with the subtropical highs (see Figure 2-1 8B , as well 
as Figure 2-21 ) and with the wavy boundary between those highs and the subpolar lows. Along 
that boundary in both hemispheres, the winds are predominantly westerly (that is, flowing from 
west to east) both at the surface and high in the troposphere. 

Embedded in the westerlies at the upper edge of the troposphere and on the boundary between 
the subpolar lows and the subtropical highs are the circumpolarjet streams, encircling the globe 
in a meandering path covering latitudes from 40° to 60°. The core of a jet stream is typically 100 
kilometers wide and 1 kilometer deep, and is characterized by wind speeds of 50 to 80 meters per 
second [1 10 to 180 miles per hour (mph)]. These high speeds are the result of a large pressure 
change over a relatively short horizontal distance on the boundary between low-pressure and 
high-pressure circulation systems. In addition to the circumpolarjet streams, there are westerly 
subtropical jet streams at about 30* north and south latitudes, associated with the poleward edges 


of the Hadley cells, and some seasonal jet streams of lesser importance. — 

On the boundary between the subpolar lows and the polar highs, there are weak easterly winds at 
the surface, giving way to westerlies at higher altitudes. The highaltitude flow, then, is entirely 
westerly. The part of this flow lying poleward of the subtropical highs is cold air ~ essentially a 
great west-to-east spinning cap of it, draped over each pole — and is called the circumpolar 
vortex. The circumpolar vortex in the winter hemisphere is larger and wavier at the edge than the 
one in the summer hemisphere, because the equator-to-pole temperature difference that is the 
basic driving force behind these features is much greater in winter. The waviness at the edge of 
the circumpolar vortex is caused by cold, low-pressure circulation systems being pushed 
equatorward into the temperate zones, producing the cold fronts and accompanying storms 
common in winter. 

In the summer hemisphere, the characteristics of the general circulation associated with the 
equator-to-pole temperature difference are much less strongly developed. The intertropical 

convergence crosses the equator into the summer hemisphere, the summer-hemisphere Hadley 
cell weakens and nearly vanishes, and the circumpolar vortex subsides. This weakening and 
relative disorganization of the general circulation in the summer hemisphere permits the pattern 
to be dominated by asymmetries connected with the distribution of land and bodies of water. 
Among the most important of these are 


Strahler and Strahler, Environmental geoscience, pp. 96-98. 

the summer monsoons of Asia and sub-Saharan Africa, in which moist, cool air sweeps inland 
over warm landmasses, rises, and drops the moisture as rain. This climatic feature is essential to 
the food supply of a substantial part of the world's population. 

Weather, Climate, and Climate Change 

Weather and climate are not the same thing, and the difference between the two terms involves 
the time span in which one is interested. Weather refers to the conditions of temperature, 
cloudiness, windiness, humidity, and precipitation that prevail at a given moment, or the average 
of such conditions over time periods ranging from hours to a few days. The weather can change 
from hour to hour, from day to day, and from week to week. 

Climate, on the other hand, means the average pattern in which weather varies in time, and the 
average is determined over longer periods (from a month to decades). Thus, one might speak of 
the climate of a given region as being characterized by hot, dry summers and severe winters. 
Within this region, one would still expect some periods (days or weeks) of cool weather in 
summer and mild weather in winter. If the weather for an entire summer were cooler than usual, 
one would still speak of an exceptional summer's weather or a short-term climatic fluctuation but 
not of climate change. But if the weather averaged over ten or twenty consecutive summers were 
significantly cooler than the average for the previous thirty, then one could begin to call the 
phenomenon a change in climate. 

Local weather and climate are determined by the complicated interaction of regional and global 
circulation patterns with local topography, vegetation, configuration of lakes, rivers, and bays, 

and so on. Successful weather prediction requires combining knowledge of these general 

patterns and known local features with the most detailed available information about the weather 
conditions of the moment and the past few days — not just at the location whose weather is being 
predicted, but at other locations, as well. That is, to predict tomorrow's weather, one must know 
as much as possible about today's. What happens in Los Angeles on Saturday may be largely 
foreseeable from what was happening in San Francisco on Friday, which could have been a 
storm that was in Portland on Wednesday, which originated as a disturbance in the Gulf of 
Alaska on Monday. 

As everyone knows, forecasting the weather even one day ahead is not an exact science. This 
inexactitude is not because there are undiscovered physical processes at work; all the basic 
physical laws involved are actually known. The imperfections in prediction have two origins: 

first, the actual system involved — atmosphere, ocean, other water bodies, land — is far too 
complicated for the known physical laws to be applied exactly, even with the largest computers; 
second, the initial conditions — the state of the system at the time the analysis begins — can be 
specified only approximately, owing to fundamental monitoring limitations. The farther in 
advance one wishes to make a local weather forecast, the more difficult the task becomes, 
because the larger is the area of the globe whose present conditions can influence subsequent 
conditions at the place one is interested in. Also, temperature and pressure anomalies small 
enough to slip through the global network of weather stations today may have grown large 
enough in a week's time to determine the weather over large regions. Weather satellites have 
made the meteorologist's task somewhat easier, especially because they provide information 
about meteorological conditions over the oceans, where surface monitoring stations are relatively 
scarce; but reliable weather forecasting a week in advance is still not a reality. Because the 
meteorological system is so complex, reliable forecasting two weeks ahead may never be 

Understanding and predicting the changes in climate that have occurred and will continue to 
occur over time spans of decades, centuries, and millennia is in some respects even more 
difficult than forecasting the weather from day to day. The weather forecaster has the 
disadvantage of having to deal with tremendous detail in terms of variations over short times and 
short distances, but the advantage of access to a tremendous body of observa- 


In this chapter we have emphasized the large-scale processes that govern the patterns of 
weather and climate over large regions, continents, and hemispheres. Investigating in detail 
the microclimates that influence intimately the human and biological communities in specific 
small regions would take us deeper into the demanding and technical subject of meteorology 
than space and the nature of this book permit. The interested reader should consult Neiburger 
et al., Understanding our atmospheric environment, or another basic meteorology text for an 
introduction to the micrometeorology missing here. 

J,- - 1 



:~rtr- irwd t*rc. iFf- *V 



f: - ■ i: ' - • 




FIGURE 2-23 The Earth's climate ~ the past million years. (After Bolin, 1974.) 
tional data — the weather happens every day, and many skilled observers with good instruments 
are watching and recording. The climatologist has reasonably good, direct, observational data 
only for the past few decades, spotty records for the past century or so, and only indirect 
evidence before that (scattered historical writings dating back several centuries, some 
archeological evidence going back a few thousand years, and only the fossil record and 
geological evidence before that). The plight of the climatologist is something like trying to learn 
all about weather on the basis of good data for the past two days, spotty data for the past two 
weeks, and only some fuzzy clues as to what might have happened before then. On the basis of 
the limited evidence available to them, climatologists have done a remarkable job of 
reconstructing in a plausible way Earth's climatic history for about the past million years and, 
more roughly, for the past 60 million years. 34 Some of the main features of the more recent 
history are illustrated in Figure 2-23 . It shows a series of fluctuations in average midlatitude air 
temperature, with different amplitudes associated with different time scales. (The amplitude of a 
fluctuation is the difference between the maximum and minimum values associated with it.) The 
amplitude of the indicated variation in the past hundred years is about 0.5° C (roughly, 1° F); the 
amplitude of the fluctuations with a time scale of a few hundred to a thousand or so years is from 
about 1.5° to 2.5° C; and the amplitude of the fluctuations with a time scale of tens of thousands 
of years is in the range of between 5° and 10° C. Several points with respect to this climatic 
history deserve emphasis: 

Variability has been the hallmark of climate over the millennia. The one statement about 
future climate that can be made with complete assurance is that it will be variable — a 
conclusion not without significance for food production (see Chapter 7). 
Rather small changes in average midlatitude temperatures are likely to be associated with 

larger changes in the seasonal extremes of temperature at those latitudes, 

34 Study of Man's Impact on Climate (SMIC), Inadvertent climate modification, pp. 28-45; Bert 
Bolin, Modelling the climate and its variations. 

and with even larger changes in the extremes nearer the poles. These extremes in 

temperature are likely to have a more crucial influence on flora and fauna than are the 


Modest changes in average temperature and the accompanying larger changes in 

temperature extremes are often associated with significant changes in the circulation 

patterns, humidities, amounts of rainfall, and other features that make up regional climates. 

These changes, too, can drastically influence the character of the plant and animal 

communities that exist in different regions. 

The drop in average midlatitude temperature associated with major ice ages is as little as 4° 

or 5° C. It is possible that an even smaller drop could trigger such an ice age. - 

The onset of significant climatic change in the past has sometimes been quite rapid. There is 

evidence to suggest, for example, that the advance of the continental ice sheets in a cooling 

period that commenced about 10,800 years ago destroyed living forests wholesale within 

the space of a single century or less. — 

The world finds itself in the last part of the twentieth century A.D. in one of the warmest 

periods in recent climatic history. What people alive today assume is normal — namely, the 

climate of the past thirty to sixty years — is in reality near one of the extremes of the 

persistent historical fluctuations. Even without the possibility of inadvertent human 

influence on climate, it could not be predicted on the basis of present knowledge how much 

longer this present extreme climate might last. Past evidence suggests, however, that when it 

ends, it will end (barring human intervention) with a cooling trend. 

Several questions present themselves. What has caused the climatic fluctuations of the past? If 
climate change is a historical fact of life, is there any reason to worry about future changes? Is 
civilization capable of inadvertently influencing climate — for example, by accelerating natural 
change or initiating a different trend? Is there any prospect of deliberate intervention to stop a 
threatening trend in climate? We consider the first two questions in the next few paragraphs; the 
last two will be touched on only briefly here and then taken up in detail in Chapter 1 1 . 

The causes of past changes in climate are not well understood. One possibility is variation in the 
rate at which the sun emitted energy, but there is no convincing evidence to show that this has 
occurred, and no convincing theory that predicts it has been proposed. ^_A second set of 
possibilities involves changes in atmospheric composition, influencing the transmission of 
incoming solar radiation and/or outgoing terrestrial radiation. For example, periods of intense 
volcanic activity might have added enough ash to the atmosphere to affect climate significantly, 
or biological and geophysical processes might have caused the atmospheric stock of carbon 
dioxide to deviate appreciably from its present value. Evidence to connect these possibilities with 
the actual onset of ice ages is lacking, however. Still another possibility is variations in Earth's 
orbit, which would have affected the amount and timing of incident solar energy. Such variations 

are known to have occurred and to be occurring, but they appear to have been too small by 
themselves to have produced the onset or retreat of ice ages. 

A likely contributing factor is that the circulation patterns and other phenomena that produce the 
gross features of climate are not very stable. If they were stable, the systems governing climate 
would tend to return after any disturbance to the conditions that prevailed before the disturbance. 
Many examples are known in physical science of systems that are stable if the disturbances 
imposed on them are not too large, but unstable — that is, they fall into altogether different 
patterns of behavior than the initial ones ~ if a disturbance exceeds some threshold. The idea of 
stability and the related concept of feedback mechanisms are explored by means of some simple 
examples in Box 2-5. 

The number of feedback mechanisms influencing the stability of the global climate, both 


positively and negatively, is very large. Some of the more important ones are indicated 

Figure 2-24 . It is not unlikely, under these circumstances, that the ocean-atmosphere 

35 SMIC. 
H. H. Lamb, The changing climate, p. 236 


For intriguing speculations, see Schneider and Mass, Volcanic dust. 
W. W. Kellogg and S. H. Schneider, Climate stabilization: for better or for worse? 

BOX 2-5 Stability and Feedback 

In general, a system is said to be stable if it tends to return to its initial state after any 
perturbation, and unstable if a perturbation would cause the system to depart permanently from 
its initial state. Often the character of the perturbation determines whether a system is stable or 

As a simple example, consider a system consisting of a marble in a round bowl, as shown in 
cross-section in the diagram here. The "state" of this system is the position of the marble, and in 
the initial state the marble is at rest at the bottom of the bowl (A). The system is stable against 
small sideways displacements of the marble -that is, if one pushes it to the side, the marble will 
roll back toward the center and eventually come to rest again in the initial state. The system is 
not stable against displacements so large that the marble is pushed out of the bowl (or against 
entirely different kinds of perturbations, such as turning the bowl over!). A similar system (B), in 
which the bowl is inverted and the marble rests on top, is unstable even against small 
perturbations. Any displacement causes the marble to roll away. 

It seems likely that the stability properties of Earth's meteorological system are analogous to 
those of the third arrangement (C). In this case, there are several states of equilibrium — that is, 
states in which the system can remain unchanged for an extended period. Each equilibrium state 

is stable against small perturbations, but a large perturbation can cause the system to shift to a 
different equilibrium, which in turn is stable until a large enough perturbation happens along to 
cause another shift. Such systems are often called metastable. 

When the forces or flows that affect the system are in balance, at least temporarily, then the 
system will be in equilibrium. A perturbation generally alters the forces or flows, and the way it 
alters them determines whether the system is stable or unstable. In a stable system, a perturbation 
sets in motion changes in forces or flows that tend to restore the initial state. Processes that work 
this way are called negative feedback (a change in the state of a system induces an effect that 
reduces the change). Positive feedback occurs when a change induces an effect that enlarges the 


Consider the marble inside the bowl again. A displacement leads to a force that pulls the marble 
back toward its initial position, reducing the displacement; this is negative feedback. If the 
marble is on top of an inverted bowl, the force that results from any displacement acts to increase 
the displacement; this is positive feedback. Cause and effect relations of this kind — cause 
producing effect that reacts back on cause — are sometimes caW^A feedback mechanisms or 
feedback loops, terms that originated in the narrower context of control systems for machinery, 
aircraft, and so on. 

We have already considered some simple feedback mechanisms in this chapter without calling 
them that. An important one for climate is the negative feedback mechanism connecting surface 
temperature and outgoing terrestrial radiation. If a perturbation should increase the surface 

temperature, this would cause the rate of energy outflow in terrestrial radiation to increase; all 
else being equal, this would cause the surface temperature to fall, reducing the initial 
perturbation. If a perturbation should reduce the surface temperature, the rate of energy loss via 
terrestrial radiation would fall, which would tend to raise the temperature again. 

Often, one must trace feedback mechanisms through two or more physical processes. Consider, 
for example, another mechanism involving Earth's surface temperature: an increase in surface 
temperature causes an increase in evaporation rate, causing increased concentration of water 
vapor in the atmosphere, causing an enhanced greenhouse effect, causing a further increase in 
surface temperature. In complicated systems such as those influencing climate, many different 
feedback mechanisms — some positive, some negative — are operating at the same time, and 
predicting the net effect of a given perturbation can therefore be very difficult. 


Solar TW&itmn 


U •■-'■: dapti 

Surface roughness 

FIGURE 2-24 Some feedback loops governing global climate. (After Kellogg and Schneider, 


system governing climate belongs to the class of physical systems that are stable against small 
disturbances but change quite drastically and semipermanently if a larger disturbance happens 
along. In the case of Earth's climate, a disturbance (such as a change in solar output or in the 
amount of solar energy reaching Earth) need not involve an energy flow larger than the flows in 
the main climatic processes in order to start an instability. It would be enough that the energy 

associated with the disturbance be sufficient to tip the balance between two competing 
feedbacks, each of which might involve a much larger energy flow than that of the disturbance. 
Such phenomena — sometimes called trigger effects — are encountered frequently in 
environmental sciences. 

There is increasing evidence suggesting that certain large variations in regional weather patterns 
are caused by a feedback effect in which anomalies in ocean surface temperatures play a major 


role. The heat capacity of the oceans (that is, the amount of energy stored for each degree the 

temperature rises) is far greater than those of the atmosphere or the land. As a result, small 
changes in the temperature of large masses of ocean water absorb or release enormous quantities 
of heat, and this permits the oceans to serve as thermal buffers, moderating what would 
otherwise be more extreme changes in seasonal temperatures in the overlying atmosphere and on 
the adjacent landmasses. That weather and climate are largely the result of the behavior of the 
ocean-atmosphere system, and not the behavior of the atmosphere alone, has been known for a 
long time. More specifically, however, 


Jerome Namias, Experiments in objectively predicting some atmospheric and oceanic 
variables for the winter of 1971-72. S. A. Farmer, A note on the long-term effects on the 
atmosphere of sea surface temperature anomalies in the north Pacific Ocean. 

recent studies indicate that weather cycles of hot and cold or wet and dry, which are observed on 
a time scale often to fifteen years in many regions, are connected with changes in ocean surface 
temperatures that persist over large areas for similar periods. Apparently, such changes can 
produce a longitudinal (east-west) shift in the wavy pattern of alternating high- and low-pressure 
zones at the edge of the circumpolar vortex. What causes the changes in ocean temperatures 
themselves is not completely understood. 

A hypothesis believed by many climatologists to explain longer-term climatic change is that the 
historical changes in Earth's orbit have been large enough to trigger a positive feedback 
involving the albedo in polar regions: reduced solar input leads to more areas being covered with 
ice and snow, leading to increased albedo (more reflectivity), leading to less solar energy 
absorbed at the surface, leading to further expansion of the ice and snow. ^Eventually, other 
(negative) feedbacks would come into play to limit the expansion of the ice and snow cover, but 
the new distribution of surface cover and associated circulation patterns — an ice age — might 
persist for thousands of years. 

Although the causes of ice ages (also called glaciations) are not well understood, there is good 
evidence concerning the actual conditions that prevailed in the Wisconsin glaciation, which was 
the most recent of several Pleistocene glaciations and ended only about 10,000 years ago. 
Compared to today's values, mean global temperatures were lowered 5° or 6° C (9° to 11° F), 
and mean temperatures were lowered by 12° C or more in the vicinity of the ice sheets 
themselves. ^_Sea level dropped at its lowest to 125 meters below today's level, the water being 
tied up in the continental ice sheets that covered what is now Canada, the north central United 
States, Scandinavia, and much of northern Europe (see Figure 2-25 ). ^_The drop in sea level 

exposed much of what is now continental shelf, which became a richly vegetated landscape. The 
snow line in most mountain areas dropped 1000 to 1400 meters, and 700 to 900 meters in 

FIGURE 2-25 Maximum extent of Pleistocene glaciation in the Northern Hemisphere. A. North 

America. B. Europe. 



SMIC, pp. 125-130; Kellogg and Schneider, Climate stabilization, p. 1 166. 

SMIC, pp. 33-34. 

Strahler and Strahler, Environmental geoscience, p. 454. 


the tropics, compressing the life zones that exist between the snow line and sea level (see 
Chapter 4). 

It is worth mentioning again that the changes producing these conditions generally may have 
been — and in some regions certainly were — quite rapid. Although one tends to think of 
glaciation as a slow process involving the plastic flow of glacial ice (as described earlier), there 
is a much faster mechanism available for the advance of glaciers during the onset of ice ages: 
snow falls over a large region, fails to melt, and is compressed under the weight of new snow the 
following winter. Thus, the area under a semipermanent cover of ice and snow can increase 
enormously in a single season. This process of rapid glaciation has aptly been termed the snow 
blitz. — 

The evidence suggests that the departure of the Wisconsin glaciers was even more sudden than 
the onset. At the peak of the warming period that followed this most recent retreat of the ice 
sheets, average global temperatures rose to 2° or 3° C warmer than today's, sea level rose to 
today's level but apparently not above it, and the prevailing circulation patterns produced 
considerably more rainfall in the Sahara and the eastern Mediterranean lands than occurs today. 

^|_These conditions, called the postglacial optimum by climatologists, occurred between 5000 
and 6000 years ago. 

The fact that Earth has a long history of climate change offers small consolation, unfortunately, 
to today's human population, faced as it is with the prospect of further change in the future. 
Significant climate change in any direction — hotter, colder, drier, wetter — in the world's major 
food-producing regions would be likely to disrupt food production for years, and even decades, 
because the animals and crops now relied upon are relatively well adapted to existing climate 
conditions. The recent historical record and the nearness of present conditions to a temperature 
maximum, moreover, suggest that the most likely major trend to occur next is cooling. This 
almost certainly would disrupt food production for as long as the lowered temperatures persisted, 
by reducing the area and growing season available for some of the most important food crops. 
The dependence of agriculture on climate is explored further in Chapter 7 and Chapter 1 1 . Other 
ecological effects of climate change could also have serious human consequences, which are 
treated in Chapter 1 1 . 

How and when human activities could themselves cause, accelerate, or prevent climate change is 
a complicated and imperfectly understood subject, which is also postponed until Chapter 1 1 . 
What is most relevant to that discussion from the foregoing treatment of the machinery of 
climate and natural climate change are the complexity and probable instability of the patterns of 
energy flow in the ocean-atmosphere system, and the speed with which changes, once triggered, 
may spread and intensify. 

Recommended for Further Reading 

Cloud, Preston, ed. 1970. Adventures in earth history. W. H. Freeman and Company, San 
Francisco. Many classic papers in the earth sciences. 

Kuenen, P. H. 1963. Realms of water. Wiley, New York. Remarkably entertaining and 
informative treatment of water in the oceans, on land, and in the atmosphere. Splended sections 
on glaciers and sea ice. 

Menard, H. W. 1974. Geology, resources, and society. W. H. Freeman and Company, San 
Francisco. Very readable text, especially good on recent climatic history. 

Nigel Calder, In the grip of a new ice age? 
44 £M/C, p. 37. 

National Academy of Sciences (NAS). 1975. Understanding climatic change. NAS, Washington, 
D.C. Excellent coverage of how climatologists reconstruct climatic history from fragmentary 
evidence, as well as of many other topics in climate. 

Neiburger, Morris; J. G. Edinger; and W. D. Bonner. 1973. Understanding our atmospheric 
environment. W. H. Freeman and Company, San Francisco. Good introductory text, with 
emphasis on weather. 

Press, Frank, and Raymond Siever. 1974. Earth. W. H. Freeman and Company, San Francisco. 
Thorough, magnificently illustrated text on the earth sciences. Emphasis on interaction of 
experiments and observations with theory. 

Strahler, A. N., and A. H. Strahler. 1973. Environmental geo science. Hamilton, Santa Barbara, 
Calif. Copious illustrations and clear explanations of an extraordinary range of topics in 
environmental earth science. 

Additional References 

Bolin, Bert. 1970. "The carbon cycle". Scientific American, September, pp. 124-132. 
Includes discussion of carbonates in the sedimentary cycle. 

1974. "Modelling the climate and its variations". Ambio, vol. 3, no. 5, pp. 180-188. 

Historical record of climate variation, and use of measurements and models to investigate 

ongoing changes. 

Bretherton, F. 1975. "Recent developments in dynamical oceanography". Quarterly Journal 

of the Royal Meteorological Society, vol. 101, pp. 705-722. Technical account of the 

emerging picture of surprisingly vigorous deep-ocean circulation, consisting of large eddies. 

Budyko, M. I. 1974. Climate and life. Academic Press, New York. Comprehensive 

technical monograph by one of the world's preeminent climatologists. 

Calder, Nigel. 1975. "In the grip of anew ice age?" International Wildlife, July/August, pp. 

33-35. Popular treatment of the snow-blitz theory. 

Cloud, Preston, and Aharon Gibor. 1970. "The oxygen cycle". Scientific American, 

September, pp. 110-123. Quite technical treatment of the evolution of Earth's atmosphere 

and the role of the sedimentary cycle in that process. 

Emery, K. O. 1969. "The continental shelves". Scientific American, September, pp. 106- 


Farmer, S. A. 1973. "A note on the long-term effects on the atmosphere of sea-surface 

temperature anomalies in the north Pacific Ocean". Weather, vol. 28, no. 3, pp. 102-105. 

Frieden, Earl. 1972. "The chemical elements of life". Scientific American, July, pp. 52-60. 

Connections between sedimentary cycle and biosphere. 

Garrels, R. M.; F. T. Mackenzie; and Cynthia Hunt. 1975. Chemical cycles and the global 

environment. Kaufmann, Los Altos, Calif. Concise and data-rich treatment of sedimentary 

cycle and chemistry of ocean and atmosphere. 

Gates, David. 1971. "The flow of energy in the biosphere". Scientific American, September, 

pp. 88-100. Interaction of climate and ecosystems. 

Isaacs, J. D. 1969. "The nature of oceanic life". Scientific American, September, pp. 146- 

162. Fascinating revelations about the fauna of the deep layers of the ocean, among other 


Kellogg, W. W., and S. H. Schneider. 1974, "Climate stabilization: For better or for worse?" 

Science, vol. 186, pp. 1 163-1 172 (December 27). Noted here for the useful discussion of 

feedback mechanisms. 

Kukla, G. J., and H. J. Kukla. 1974. "Increased surface albedo in the Northern Hemisphere". 

Science, vol. 183, pp. 709-714 (February 22). Changes in snow and ice cover in the 
Northern Hemisphere were great enough in the early 1970s to cause significant changes in 
the hemispheric heat balance. Satellite measurements are reported. 

Lamb, H. H. 1966. The changing climate. Methuen, London. Methods and conclusions of 
historical climatology. 

1972. Climate: Past, present, and future. Methuen, London. Excellent introduction to 


Leopold, Luna B. 1974. Water. W. H. Freeman and Company, San Francisco. Readable 
introduction to the hydrologic cycle. 

Lorenz, E. N. 1970. "Climate change as a mathematical problem". Journal of Applied 
Meteorology, vol. 9, pp. 325-329. The author argues that climate change could be caused by 
"internal" fluctuations that are characteristic of complicated physical systems. 
McDonald, James E. 1952. "The coriolis effect". Scientific America, May. Reprinted in J. R. 
Moore, ed., Oceanography, pp. 60-63. Readable elaboration and examples. 
Maclntyre, Ferren. 1970. "Why the sea is salt". Scientific American, November, pp. 104- 
115. Excellent introduction to geochemistry. 

Mason, Brian. 1966. Principles of geochemistry. 3d ed. Wiley, New York. Good 
introductory text. 

Miller, Albert. 1966. Meteorology. Merrill, Columbus, Ohio. Concise and readable 

Moore, J. R., ed. 1971. Oceanography: Readings from Scientific American. W. H. Freeman 
and Company, San Francisco. Collection of articles. 

Munk, Walter. 1955. "The circulation of the oceans". Scientific American, September. 
Reprinted in J. R. Moore, ed., Oceanography, pp. 64-69. 

Namias, Jerome. 1969. "Seasonal interactions between the north Pacific Ocean and the 
atmosphere during the 1960s". Monthly Weather Review, vol. 97, no. 3, pp. 173-192. Early 
analysis of a possible controlling role for the ocean in climate fluctuations. 

1972. "Experiments in objectively predicting some atmospheric and oceanic variables 

for the winter of 1971-2". Journal of Applied Meteorology, vol. 11, no. 8, pp. 1164-74. 

Oort, Abraham H. 1970. "The energy cycle of the earth". Scientific American, September, 

pp. 54-63. Sizes and functions of major natural energy flows. 

Rona, P. A. 1973. "Plate tectonics and mineral resources". Scientific American, July. 

SCEP. SeeStudy of Critical Environmental Problems. 

Schneider, S. H.; and Roger D. Dennett. 1975. "Climatic barriers to long-term energy 

growth". Ambio, vol. 4, no. 2, pp. 65-74. Contains good synopsis of knowledge about the 

operation of global climatic machinery. 

Schneider, S. H., and C. Mass. 1975. "Volcanic dust, sunspots, and temperature trends". 

Science, vol. 190, pp. 741-746 (November 21). Interesting speculations and analysis 

centered around the influence on climate of possible variations in the sun's output. 

Sellers, William D. 1965. Physical climatology. University of Chicago Press. Excellent text. 

Siever, Raymond. 1974. "The steady state of the earth's crust, atmosphere, and oceans". 

Scientific American, June, pp. 72-79. An illuminating review of the main ideas in 


Skinner, Brian J. 1969. Earth resources. Prentice-Hall, Englewood Cliffs, N.J. Capsule 

introduction to the hydrologic cycle, among other topics. 

SMIC. SeeStudy of Man's Impact on Climate. 

Stewart, R. W. 1969. "The atmosphere and the ocean". Scientific. American, September, pp. 
76-86. Useful summary of some ocean-atmosphere interactions, although dated somewhat 
by recent developments 

(see Jerome Namias, "Seasonal interactions between the north Pacific Ocean and the 
atmosphere during the 1960s"; 

Namias, "Experiments in objectively predicting"; and F. Bretherton, "Recent developments 
in dynamical oceanography"). 

Study of Critical Environmental Problems (SCEP). 1970. Man's impact on the global 
environment. M.I.T. Press, Cambridge, Mass. Contains short summaries of main global 
climatic processes. 

Study of Man's Impact on Climate (SMIC). 1971. Inadvertent climate modification. M.I.T. 
Press, Cambridge, Mass. A wealth of information and references on the study of climate and 
climate change. Moderately technical. 

Wenk, Edward Jr. 1969. "The physical resources of the ocean". Scientific American, pp. 

Wilson, J. Tuzo, ed. 1972. Continents adrift: Readings from Scientific American. W. H. 
Freeman and Company, San Francisco. Articles on plate tectonics and continental drift. 
Woodwell, George M. 1970. "The energy cycle of the biosphere". Scientific American, 
September, pp. 64-74. 

Wyllie, Peter J. 1975. "The earth's mantle". Scientific American, March, pp. 50-63. Plate 
tectonics, earthquakes, and the composition of the planet. 

[This page intentionally left blank.] 


Nutrient Cycles 

All things from eternity are of like form and come round in a circle. 

~ Marcus Aurelius 

The main flow of energy that helps shape conditions on Earth's surface comes from space; and 
when the energy's work here is done, to space it returns. With respect to energy, then, Earth is an 
open system. With respect to its chemical endowment, however, Earth is a closed system. That 
is, the amounts of carbon, hydrogen, oxygen, iron, gold, and other elements in the planet- 
atmosphere system do not change with time; the chemical arrangement and physical distribution 
of these elements can and do vary, but essentially nothing enters and nothing leaves the system. - 

The elements in this closed system that are essential to life are called nutrients. They can be 
divided into three categories: the 4 main chemical building blocks of living matter (carbon, 
oxygen, hydrogen, nitrogen); 7 macronutrients, of which smaller but still significant quantities 

are required for life; and 13 micronutrients, or trace elements, of which tiny quantities perform 
essential functions (see Table 3-1 ). iThe suitability of any terrestrial or aquatic environment for 
the support of life depends on the availability of nutrients in appropriate forms and quantities. 
The processes that govern this availability (or lack of it) are known collectively as nutrient 
cycles, because of the way the individual basic stocks of physical material move cyclically 
through the living and nonliving parts of the physical world. 

These cycles function in support of life not merely by making nutrients continuously available — 
in other 

! The main exceptions are some hydrogen escaping into space from the top of the atmosphere, 
some hydrogen arriving in the "solar wind" from the sun, and some iron and other elements 
arriving in the form of meteorites. The quantities involved in these flows are negligible for the 
purposes of this discussion. 
Frieden, The chemical elements of life. 

words, by maintaining the fertility of the environment -but also by limiting the accumulation of 
material in quantities, forms, and places in which it would damage organisms. It is important to 
recognize in this connection that the same elements and compounds that serve as nutrients for 
some organisms in some concentrations are often toxic for other organisms, or even for the same 
organisms at higher concentrations. For example, molecular oxygen (O 2) is toxic to anaerobic 
organisms and, at high enough concentrations, even to mammals. Ammonia (NH 3 ) is an 
important source of nutrient nitrogen for many plants but is toxic to people. Hydrogen sulfide (H 
2 S) is a nutrient for certain types of bacteria but is extremely toxic to mammals. 

TABLE 3-1 The Chemical Elements of Life 
Symbol Name 

H Hydrogen 

C Carbon 

N Nitrogen 

O Oxygen 


Na Sodium 

Mg Magnesium 

P Phosphorus 

S Sulfur 

CI Chlorine 

K Potassium 

Ca Calcium 


Atomic number 

Atomic weight 

























Atomic number Atomic weight 





















































Source: E. Frieden, The chemical elements of life. 


The steps that make up all nutrient cycles involve two basic processes: physical transport and 
chemical transformation. 

The principal agents of physical transport are discussed in Chapter 2. They are: (1) the 
hydrologic cycle, including evaporation and precipitation as well as the flow of streams, 
groundwater, and ice; (2) winds; (3) ocean currents; and (4) geologic movement, especially the 
upward and downward motions at the boundaries of tectonic plates and geologic uplifting on 
continents. A fifth agent of physical transport that is important in a few circumstances is the 
movement of organisms. For example, fish-eating birds that deposit their excrement on land 
provide a significant pathway by which nitrogen and phosphorus are transferred from the sea to 
the land. Fish such as salmon, that feed mostly in the ocean but migrate up freshwater rivers to 
spawn and die, perform a similar function, as do the ocean-caught fish consumed by continent- 
dwelling human beings. 

How the various mechanisms of transport link the principal "compartments" of the physical 
world is represented schematically in Figure 3-1 . Obviously, the ease (and, hence, the rate) with 
which a given substance can move through these pathways depends on its physical state (solid, 
liquid, gas), on other physical properties (solubility, volatility), and on the possibility of chemical 
transformations that can change those properties. Such chemical transformations take place in 
living organisms and in the nonliving environment. They are considered in more detail below. 
The magnitudes of the flows in some of the principal pathways shown in Figure 3-1 are listed in 
Table 3-2. 

Each of the compartments indicated in Figure 3-1 often is divided into subcompartments for the 
purpose of tracing nutrient flows. Thus, for example, it is customary to consider three 
subcompartments of material in both the soil and the oceans: living organic matter, dead organic 

matter, and the inorganic medium itself. These subcompartments can be further subdivided — for 
example, organic matter, into plants, animals, and microorganisms; and seawater, into the well- 
mixed surface layer and the stratified deeper layers. Finally, within each subdivision of each 
subcompartment, the magnitudes of 



















FIGURE 3-1 Transport mechanisms among the "compartments" of the physical world. 

the stocks, or pools, of the important elements and compounds are of interest, as are the 
magnitudes of the flows into and out of the pools (where flows can mean physical movement or 
chemical transformation or a combination of the two). 

For any pool that is in steady state ~ that is, for which inflows balance outflows so the size of the 
pool does not change with time ~ an average residence time for a compound can be calculated as 
the ratio of the size of the pool to the sum of the inflows or outflows (see Box 2-3). For example, 

residence time (yr) = 

pool (kg) 
sum of outflows (kg/yr) 

Examination of the residence times for important pools provides quick insight into the dynamics 
of nutrient cycling, and, indirectly, into the probable vulnerability of specific components of 
nutrient cycles to disruption by human intervention. For example, a long residence time means 
the annual flows are small compared to the pool, which means in turn that unless human 
disruptions cause 

TABLE 3-2 Some Global Materials Flows 



River discharge (H 2 O) 32,000 

Groundwater flow (H 2 O) 4,000 

River-borne suspended solids (today) 1 8 

River-borne suspended solids (prehuman) 5(?) 

River-borne dissolved solids 4 

Ice-sheet transport of solids 2 

Shoreline erosion 0.3 


Evaporation (H 2 O) 526,000 

CO 2 exchange in photosynthesis 220 

CO 2 from burning fossil fuels 1 6 

Dust and smoke from land 1 (?) 

Volcanic gases and debris 0.1 

Note: Uncertainty in most estimates is roughly ±20 percent. 

Sources: R. M. Garrels, F. T. Mackenzie, and C. Hunt, Chemical cycles 
and the global environment; Study of Man's Impact on Climate, Inadvertent 
climate modification. 



of flow 

(billion MT/yr) 

TABLE 3-3 

Residence Times in Some Important Nutrient Pools 

Nutrient pool 

Nitrogen in atmosphere 

Oxygen in atmosphere 

Inorganic nitrogen in soil 

Carbon in dead terrestrial organic matter 

Residence time (yr) 





Nutrient pool Residence time (yr) 

Carbon in living terrestrial organic matter 17 

Carbon dioxide in atmosphere 5 

Carbon in living oceanic organic matter 0.10 

Sulfur compounds in atmosphere 0.02 
Note: Uncertainties are at least ±20 percent. 

a very large alteration in inflow or outflow they will affect the size of the pool only very slowly. 
A short residence time, by contrast, means that significant changes in inflow or outflow can 
affect the size of the pool very quickly. Approximate residence times for some of the important 
nutrient pools appear in Table 3-3 . 


Two kinds of chemistry play crucial roles in the principal global nutrient cycles: the chemistry of 
the sedimentary cycle, associated with the very slow transformation of very large pools of 
material, and the chemistry of life, associated with the more rapid transformation of much 
smaller pools. Both kinds of chemistry depend very heavily on the very special roles of water as 
a source of hydrogen (H+) and hydroxyl (OH-) ions for many chemical reactions, as a medium 
for holding other chemical compounds in solution where they can react, and as a vehicle for 
physical transport of compounds from place to place. 

Sedimentary Cycle 

Nutrients are mobilized in the sedimentary cycle when water comes in contact with rock. 
Rainwater is slightly acidic, having absorbed some atmospheric carbon dioxide to form weak 
carbonic acid: - 

H 2 + C0 2 > H 2 C0 3 

H 2 C0 3 > H+ + HCOj- 

The pH of rainfall in areas free of industrial pollution is around 5.7. (Recall from basic chemistry 
that pH is the negative of the log to base 10 of the concentration of hydrogen ions in moles per 
liter. The pH of distilled water is 7.0, corresponding to 10-7 moles of hydrogen ion per liter.) 
This slight acidity is enough to facilitate greatly the dissolution of many minerals present in 
exposed rock — a process called weathering. 

Among the most important weathering reactions is that of the common mineral feldspar (a 
constituent of granite): 

feldspar carbonic acid water 

2KAlSi 3 8 + 2(H + + HC0 3 -) + H 2 » 

dissolved dissolved potassium 

kaolinite silica and bicarbonate ion 

Al 2 Si 2 5 (OH), + 4Si0 2 + 2K+ + 2HCO 


Kaolinite is a clay; it is left behind in this process as a contribution to the formation of soil, 
whereas the dissolved silica, potassium ion, and bicarbonate ion are carried away in the runoff. 
The mobilization and removal of positive ions such as (in this case) potassium is called leaching. 
Note that the reaction consumes some water and that it reduces the acidity of the solution (that is, 
it removes hydrogen ions) as it proceeds. These properties are characteristic of weathering 
reactions. Essentially the same reaction also occurs in the weathering of the aluminosilicates of 
sodium and calcium (NaAlSi308, CaA12Si208). Other silicate minerals also weather in the 
same general manner to produce kaolinite or other clays. 

The weathering of certain other minerals leads to their complete dissolution; no solid trace is left 
behind to be incorporated into soil. The most important of these minerals are the limestones, or 
carbonate rocks, which undergo the following reactions: 

CaCOj + H+ + HCO," > Ca + + + 2HCOj" 

CaMg(COj) 2 + 2(H+ + HCO,-) » 

Mg ++ + Ca++ + 4HC0 3 - 

Still other weathering reactions consume atmospheric oxygen in the oxidation of iron minerals: 

4FeSiOj + 2 + 2H 2 » 4FeO(OH) + 4Si0 2 

4Fe 3 4 + 2 ► 6Fe 2 3 

4FeS 2 + 150 2 + 8H 2 ► 2Fe 2 5 + 8H 2 S0 4 


A good discussion of this point is given by F. Press and R. Siever in Earth, Chapter 6. 

Most of the products of the weathering reactions mentioned here eventually reach the sea, either 
as dissolved ions or as suspended solids. Between the time they are freed from rock and the time 
they reach the sea, of course, some of these elements have made one or more passages through 

the living part of the biosphere as nutrients. In the sea, too, the nutrient substances are captured 
from the water by living organisms, eventually to be returned to the water or to the sediments on 
the ocean floor. The reverse of many of the weathering reactions also can occur in seawater in 
the absence of intervention by living organisms, forming solid precipitates that join the organic 
debris on the ocean bottom. Apparently these outputs of the major nutrient elements from the 
oceans roughly balance the inputs, because considerable physical evidence indicates that the 
salinity of seawater has been approximately constant for at least the past 200 million years. - 

The sedimentary cycle is finally closed by the geologic processes described in Chapter 2: that is, 
by the processes of transformation and transportation that turn seafloor sediments into 
continental rocks, and by volcanic action. 

Living Matter 

Ecologist Edward S. Deevey, Jr., has pointed out that living matter can be regarded as a chemical 
compound with the empirical formula - 

H 2960 O 1480 C 1480 N 16 P 1.8 S. 

This enormous (and completely intentional) oversimplification serves both to call attention to the 
chemical side of life ~ the side with which this chapter is concerned -and, more specifically, to 
illustrate the relative proportions in which the six most abundant elemental constituents of living 
material exist. 

Actually, there is substantial variation in the ratios of the chemical elements of life in different 
kinds of organisms, the above formula being a weighted average in which woody plants 
predominate. The numbers are, in any case, not known with great accuracy. Another source gives 
the molar ratio H/O/C/N/S/P as 1600/800/800/9/5/1 for land plants (different from Deevey 
mainly in the abundances of sulfur and phosphorus) and 212/106/106/16/2/1 for living marine 
plants, soil humus, and organic materials in sedimentary rocks. ^(Readers who have not studied 
basic chemistry or who have forgotten it should see Box 3-1 for a review of mole and mass 

The basic reactions that link the water cycle, the carbon dioxide cycle, and the oxygen cycle 
through living matter are photosynthesis: 

6 C0 2 + 12 H 2 + solar energy > 

+ 6 2 + C 6 H l2 6 + 6H 2 

and its opposite, respiration, or oxidative metabolism: 

6 2 + C 6 H 12 6 > 6 C0 2 + 6 H 2 + heat 

The compound (C 6 H n O 6, or (CH 2 o ) 6, is glucose, the simplest carbohydrate produced by 
photosynthesis. The photosynthesis reaction is written with what appear to be six extra H 2 O 

molecules on each side because twelve molecules of H 2 O are actually broken up for each 
molecule of C 6 H u O 6 synthesized; radioactive-tracer experiments show that all twelve oxygen 
atoms from the 12 H 2 O end up in the 60 2, and the oxygen in the six new water molecules 
comes from the CO 2- _The amount of energy transformed from light into stored chemical 
energy by the photosynthesis reaction is 112 kilocalories (469 kilojoules) per mole of carbon 
fixed. The net result of the respiration reaction is to transform the same 112 kilocalories per mole 
of carbon from chemical energy into heat. 

Photosynthesis is carried out not only by higher green plants (such as oak trees, grasses, and 
ferns) but also by various kinds of algae (green, brown, red, blue-green) and by certain bacteria. 
In the bacteria, substances other than water serve as the sources of hydrogen for the reduction of 
carbon. _For example, the green sulfur 

Ferren Maclntyre, Why the sea is salt. 
Edward S. Deevey, Jr., Mineral cycles. 

R. M. Garrels, F. T. Mackenzie, C. Hunt, Chemical cycles and the global environment. 
7 See, for example, A. L. Lehninger, Biochemistry, p. 457. 

Reduction is the term for gaining electrons in chemical reactions, to be contrasted with 
oxidation, which means losing electrons. In most cases, gaining a hydrogen atom is equivalent 
to gaining an electron, because hydrogen effectively gives up its single electron to most 
elements with which it forms compounds. See, for example, L. Pauling, General chemistry, or 
any other basic chemistry text. 

BOX 3-1 Moles, Molecules, and Mass Ratios 

Amole of an element is 6.02 x 10 atoms; a mole of a compound is 6.02 x 10 molecules. This 
number ~ known as Avogadro's number — is the number of atomic mass units in a gram. (The 
atomic mass unit is defined as one-twelfth of the mass of the most abundant isotope of carbon, 
which is carbon- 12. One atomic mass unit is approximately the mass of a proton.) It follows 
from these definitions that a mole of any element or compound ~ 6.02 x 10 atoms or molecules 
of it — has a mass in grams equal to the atomic or molecular weight of the substance in atomic 
mass units. For example, a mole of carbon (atomic weight, 12 atomic mass units) has a mass of 
12 g; a mole of carbon dioxide, or CO 2 (atomic weight, 44 atomic mass units), has a mass of 44 


The mass ratios of elements in living matter and in other compounds differ from the molar ratios 
because different elements have different atomic weights. Consider the carbohydrate molecule 
(CH 2 O) n , where n is an integer telling how many times the basic CH 2 O building block appears 
in the molecule. The ratio of moles of hydrogen to moles of carbon to moles of oxygen in this 
material is 2 to 1 to 1 — in shorthand, H/C/O = 2/1/1. The ratio of the masses is obtained by 
multiplying the numbers in the molar ratios by the appropriate atomic weights. The mass ratio 
H/C/O in (CH 2 0)n is (2 x 1)/ (1 x 12)/(1 x 16) = 2/12/16 = 1/6/8. 

bacteria use hydrogen sulfide instead of water, according to the equation 

12 H 2 S + 6 C0 2 + light 

C 6 H 12 6 + 6 H 2 + 12 S 

producing elemental sulfur rather than oxygen. (In fact, these bacteria are obligate anaerobes — 
they would be poisoned by oxygen.) The reverse reaction (respiration) consumes sulfur and 
carbohydrate and liberates heat, carbon dioxide, and hydrogen sulfide. Still other photosynthesis 
reactions use hydrogen, obtained from H 2 O or H 2 S or other hydrogen donors, to reduce 
substances other than carbon dioxide. For example, most higher plants can reduce nitrate, instead 
of CO 2, in photosynthesis: 

9 H 2 + 2 NOj- + light » 

2 NH, + 6 H 2 + 4± 2 

and nitrogen-fixing photosynthetic organisms can reduce atmospheric nitrogen: - 

6 2 + C 6 H, 2 6 » 6 C0 2 + 6 H 2 + heat 

These variations notwithstanding, the great bulk of photosynthesis on Earth consists of the 
reduction of carbon dioxide, with water as the hydrogen source. 

The photosynthesis and respiration reactions, as we have written them here, actually represent 
only the end points of vastly more complicated sequences of chemical events. For example, 
although burning glucose in air is described essentially completely by the respiration reaction 
given above, 

60 2 +C 6 Hi2 06^6C0 2 +6H 2 O + heat, 

to achieve the same result in a living cell (at a temperature far below that of combustion) requires 
many intermediate steps. The intermediate steps in both respiration and photosynthesis rely 
heavily on three kinds of organic compounds: the energy carrier ATP (adenosine triphosphate); 
the electron carrier NADP (nicotinamide adenine dinucleotide phosphate); and various catalysts 
called enzymes. (Catalysts speed up chemical reactions without themselves being consumed.) — 

A closer look at these compounds provides some clues to the importance of the elements 
nitrogen, phosphorus, and sulfur in the chemistry of life. All biochemical reactions of importance 
(not just photosynthesis and respiration) require enzyme catalysts if they are to proceed at 
significant rates. All enzymes are proteins. Proteins other than enzymes also serve as hormones; 
as vehicles to transport oxygen; as important structural 

10 For detailed descriptions of the biochemistry of photosynthesis and respiration see R. P. 

Levine, The mechanism of photosynthesis; P. Cloud and A. Gibor, The oxygen cycle; 
Lehninger, Biochemistry. 

9 Lehninger, Biochemistry; and R. Y. Stanier, M. Douderoff, and E. A. Adelberg , The 
Microbial world, Prentice-Hall, Englewood Cliffs, N.J., 1970 . 

components of skin, tendons, muscle, cartilage, and bone; and in numerous other roles. All 
proteins are made up of building blocks called amino acids, and all amino acids contain the 
amino group — NH 2. A single protein contains from fifty to some tens of thousands of amino 
acids, and hence at least that many nitrogen atoms. (Some amino acids contain more than one 
nitrogen.) The shape and stiffness of many proteins — properties essential to their functions — are 
governed and maintained by bonds in which sulfur plays a crucial role, so it, too, is essential to 
the chemistry of life. 

ATP and NADP are examples of nucleotides, compounds consisting of a five-carbon sugar to 
which one or more phosphate groups ( — P04) and a nitrogen base are attached. A nitrogen base 
is a ring compound containing both nitrogen and carbon in the ring structure. Nucleotides not 
only perform critical energy-transfer and electron-transfer functions in the chemistry of living 
cells, but they are also the building blocks of nucleic acids. Each nucleic acid molecule contains 
hundreds to thousands of nucleotides, the arrangement of which is a chemical means of storing 
information. Nucleic acids control the synthesis of proteins and a variety of other processes in 
living cells. The nucleic acid DNA (deoxyribonucleic acid) is the genetic material — the bearer 
of the coded information that is passed from parent cells and organisms to their offspring to "tell" 
them what to become. The nucleic acid RNA (ribonucleic acid) plays essential roles in carrying 
out the instructions coded in DNA molecules. — 

In addition to its role as a constituent of DNA and RNA, phosphorus, in the form of phosphate, is 
one of the principal anions (ions with a negative charge) that maintains electric neutrality in the 
fluids of living organisms. Along with calcium, it is also an important component in shells and 
bony structures. Sulfur, besides playing its role in proteins, in the form of sulfate( — SO 4) is 
another principal anion in body fluids and cells. The third principal anion is the chloride ion ( — 
CI). The principal cations (positively charged ions) are sodium, potassium, calcium, and 
magnesium. Magnesium is an essential constituent of many enzymes and of the green pigment 
chlorophyll. Iron is a key component of many enzymes and of the oxygen-transporting protein 

1 n 

hemoglobin. — 

Oversimplifying considerably, one can say that the quantity of living matter in a given 
environment is limited by the stock of that requisite of life that is in shortest supply. This rather 

obvious proposition carries the name Liebig's law of the minimum. Of course, the limiting 

factor varies from one type of organism to another and from one environment to another. 
Generalizations are difficult, but it is fair to say that under many circumstances the quantity of 
plant material in a given environment is in fact controlled by a limiting nutrient (although the 
density of a given kind of plant is often controlled by a herbivore). 14 In desert regions the 
limiting nutrient is generally water. Under many conditions where water is not limiting, as in 
freshwater lakes, the limiting nutrient turns out to be phosphorus; in the open ocean it may be 
iron. In some richly productive environments, nitrogen and phosphorus are jointly limiting — 

producing any more plant material would require supplying more of both nutrients. Obviously, 
the limiting nutrient in a given biological system represents a leverage point where the effects of 
natural or human-induced perturbations manifest themselves rapidly. 


The cycles of many nutrients are tightly linked chemically and biologically. This is so much the 
case for the cycles of carbon, oxygen, and hydrogen that we consider them together. The cycles 
of nitrogen, phosphorus, and sulfur, on the other hand, can usefully be examined separately, as 
they are here. 

n For an introductory discussion, see Paul R. Ehrlich, R. Holm, M. Soule , Introductory biology. 
More detail is available in J. D. Watson, The molecular biology of the gene. 

1 9 

Material in this paragraph is treated more fully in Frieden, Chemical elements. 
For a more thorough discussion see E. P. Odum, Fundamentals of ecology. 
For example, Paul R. Ehrlich and L. C. Birch, The "balance of nature" and population 

TABLE 3-4 

Phytomass and Net Primary Production on Land and in the Oceans 

Phytomass Estimated net primary production 

On Land 



Note: MT = metric tons, conversions from mass to energy at 0.45 gram carbon per gram dry 
organic matter, 10 kcal per gram carbon. For ranges of uncertainty, see text. 

Sources: After R. H. Whittaker and G. E. Likens, Carbon in the biota, in Carbon and the 

biosphere, G. H. Woodwell and E. V. Pecan, eds. 


It is appropriate to begin our closer examination of the carbon-oxygen-hydrogen cycles with 
their intersection in the green plant, the basis of almost all life on Earth. 

Carbon and energy in living plants. Most living plants not only fix (store) energy by means of 
photosynthesis but they also use some of that stored energy to drive their own life-sustaining 
internal processes by means of respiration. The total rate at which a plant or a plant community 
stores solar energy by means of photosynthesis is called gross photosynthesis or gross primary 
production (GPP). The figure obtained by subtracting from gross photosynthesis the amount of 
energy used by the plants in respiration (R) is called net photosynthesis or net primary 
production (NPP). In abbreviated form, then, the relation is GPP - R = NPP. Several 

(10 9 MT, dry 


(10 9 


MT dry organic 
matter /yr) 


(10 moles 

(10 ls kcal/yr) 


(10 u watts) 












interchangeable units are used in the biological literature for the measurement of these quantities: 
calories or kilocalories per unit of time, watts or kilowatts (different units for measuring the same 
thing — energy flow), grams of carbon fixed into organic compounds per unit of time, moles of 
carbon fixed per unit of time, or grams of dry organic matter produced per unit of time. The 
relations among these units are discussed in Box 3-2. 

A wide range of estimates of global net primary production and the associated standing crop of 
living plants (phytomass) have been published. ^_One recent review indicates that the estimates 
of net primary production are converging on a value around 160 billion (1.60 x 10 11 ) metric tons 
per year for the globe. ^Global phytomass was estimated in the same review as about ten times 
larger — 1840 billion metric tons, dry weight — and gross photosynthesis was estimated to be 
about twice as large as net production (that is, half of gross photosynthesis is consumed in plant 
respiration and half remains as net production). The estimates for phytomass and net primary 
production, separated into terrestrial and oceanic components, are presented in Table 3-4 . 

The roughness of these estimates deserves emphasis. The range of recent ( 1970 or later) 
estimates of land phytomass, for example, is from about half the value given in Table 3-4 to 30 
percent larger; the range of published values of ocean phytomass is from twenty times smaller 

1 7 

than the value in Table 3-4 to three times larger. The factor-of-60 range of informed opinion 

with respect to oceanic phytomass is especially remarkable. The range in recent published 
estimates of oceanic net primary production is only ±10 percent from the value in Table 3-4 , and 
the range in terrestrial net production is from 15 percent less to 60 percent more than the value in 

the table. The fraction of gross primary production consumed in plant respiration is known to 

vary between 20 percent and 75 percent, depending on type of plant and region, but the global 
average is unlikely to differ much from 50 percent. — 

15 See especially, G. M. Woodwell and E. V. Pecan, eds., Carbon and the biosphere; United 
States National Committee for the International Biological Program, Productivity of world 

16 R. H. Whittaker and G. E. Likens, Carbon in the biota, Carbon and the biosphere, Woodwell 
and Pecan, pp. 281-300. 

1 7 

The high value of terrestrial phytomass and the low value of oceanic phytomass are from L. 
Rodin, N. Bazilevich, and N. Rozov, Productivity of the world's main ecosystems. The 
opposite extremes are found in B. Bolin , The carbon cycle. 
Whittaker and Likens, Carbon, p. 291. 
R. H. Whittaker, Communities and ecosystems. 

BOX 3-2 Units for Energy and Material Flow in the Carbon Cycle 

For the purpose of reconciling estimates of annual production presented in grams of carbon, 
grams of dry organic matter, and kilocalories, one widely used convention assumes that 10 
kilocalories of energy are stored per gram of fixed carbon and that dry organic matter contains 45 
percent fixed carbon by weight. (The actual range of values for different kinds of dry organic 
matter extends roughly ±15 percent from these nominal figures.) Thus, measurements in grams 

or metric tons (1 MT = 106 g) of carbon can be converted to grams or metric tons of dry organic 
matter by multiplying by 2.2, and 1 gram of dry organic matter can be taken to represent 4.5 
kilocalories of stored energy. Useful conversions following directly from this convention are that 
I gram of dry organic matter per day corresponds to an average energy flow of about 0.2 watts; 1 
gram of carbon per day, to about 0.5 watts; and one metric ton of dry organic matter per year, to 
about 600 watts. Also, 1 metric ton of dry organic matter contains 37,500 moles of carbon. 

Although we shall use the convention just described, the reader may encounter others elsewhere. 
If the dry organic matter were pure carbohydrate — (CH 2 0)n — which stores 1 12 kilocalories 
per mole of carbon, then the appropriate conversions would be: 3.7 kilocalories per gram of dry 
organic matter, 9.3 kilocalories per gram of carbon, 0.4 gram carbon per gram of dry organic 
matter. [Actually, CH 2 O is a good approximation of the average composition of all living matter 
(see Deevey's formula presented earlier) because so much of the world's biomass is cellulose tied 
up in woody plants, and cellulose has nearly this formula. However, most of the plant material 
produced each year is not cellulose — remember, a big pool does not necessarily mean a big 
flow. The annual production of organic matter contains a higher proportion of fixed carbon and 
higher energy content per gram than does the cellulose-dominated, slow-toturn-over standing 
crop.] The rounded-off value of 4 kilocalories per gram of dry organic matter is in fairly 
widespread use in conjunction with the figure of 10 kilocalories per gram of fixed carbon, itself a 
rough average among several kinds of plant matter. 

These differences in conversion values are troublesome for those seeking consistency in the 
ecological literature, but the differences they produce in the published figures for net production 
are much smaller than the uncertainties of measurement and estimation. 

See, for example, R. H. Whittaker and G. E. Likens, "Carbon in the biota", in Carbon and the 
biosphere, G. H. Woodwell and E. V. Pecan, eds., pp. 281-300. 

Taking the range of estimates into account, one can state several conclusions with reasonable 
assurance: (1) the annual NPP of the oceans is only one-third to one-half that of the continents, 
even though the oceans cover more than 70 percent of Earth's surface; (2) the pool, or standing 
crop, of living plant material on land is at least several hundred times that in the oceans; (3) the 
average turnover time (life expectancy) of oceanic plants must be in the range of days to weeks, 
in order for so small a phytomass to be associated with so large an annual production; (4) the 
average turnover time of terrestrial living plants, by contrast, is in the range of a decade or two; 

(5) oceanic and terrestrial phytomasses are both so poorly known that man-induced or natural 

changes large enough to be very important in the long term could easily escape notice for some 
time. In the latter regard, although there are good reasons to suppose that the activities of 
civilization have diminished the terrestrial phytomass significantly, some experts consider this 

7 1 

assertion to be debatable. — 

Notwithstanding the uncertainties in the data, it is instructive to examine the large regional 
variations in plant productivity that are concealed in the global totals. Some of these figures (on 
which the totals in Table 3-4 are based) are summarized in Table 3-5 . Of particular interest are 
the facts that tropical forests account for more than 40 percent of terrestrial NPP (although they 
cover only about 16 percent of the land area) and that up- 


Obviously, this longevity is strongly influenced by the high proportion of the terrestrial 
biomass in long-lived trees and shrubs. Most crop plants, of course, "turn over" in less than a 
21 See the comments in Whittaker and Likens, Carbon, pp. 295-297, and the group discussion 
stimulated by them (pp. 300-302). 

TABLE 3-5 

Variation and Distribution of Net Primary Production 

Range ofNPP Total NPP 

(g organic matter /m /yr) & - 1 - (billion MT/yr) - 








Tropical Forest 


Shrubland, Grassland, Savanna 


Temperate Forest 


Northern Forest 


Cultivated Land 


Swamps, Lakes, Marshes 


Desert, Tundra, Alpine Meadow 


Terrestrial total 

Open Ocean 


Continental Shelf 


Reefs, Estuaries 


Upwelling Regions 







Oceanic total 55.4 

a R. H. Whittaker and G. E. Likens, Carbon in the biota, in Carbon and the biosphere, G. H. 
and E. V. Pecan, eds., p. 283. 

L. Rodin, N. Bazilevich, and N. Rozov, Productivity of the world's main ecosystems, pp. 15-17, 


c G. H. Woodwell, The energy cycle of the biosphere, Scientific American, September 1970 , pp. 


welling regions, continental shelves, reefs, and estuaries yield almost 25 percent of oceanic NPP 
while accounting for less than 9 percent of the ocean's area. 

According to Table 3-4 , the rate at which energy is being stored in net primary production is 
about 100 million megawatts, roughly two-thirds on land and one-third in the ocean. The rate of 
gross primary production can be assumed to be about 200 million megawatts. The net primary 
production is roughly twelve times the rate at which civilization used commercial energy in 

1975, where commercial energy refers to energy that is sold (for example, fossil fuels, 
hydropower, and nuclear power, but excluding food) as opposed to energy that is gathered by the 
user (for example, most wood, dung). 

Very little of the solar energy that reaches Earth's surface is captured by photosynthesis. The net 
primary production is about 0.1 percent of the incident solar energy at the surface-0.25 percent of 
what strikes the land and 0.05 percent of what strikes the ocean. Some of the reasons these 
percentages are so small are obvious. Plants do not by any means cover all of Earth's surface; in 
many areas of the world the growing season lasts only part of the year; and in some 
circumstances, as already noted, plant growth is limited by lack of one or more critical nutrients. 
Under favorable conditions, on the other hand — as in a well watered and fertilized cornfield or a 
fertile swamp — NPP may reach 2 percent or so of incident solar energy averaged over the 
growing season and as much as 6 percent or 7 percent of incident solar energy on the most 
favorable days. — 

These numbers still seem low. One additional reason for this is that only about one-fourth the 
solar energy reaching the ground is in the part of the wavelength range that stimulates 

photosynthesis (blue and red light). The other big reason is the large amount of water that must 

be evaporated through the leaves of plants to maintain the nutrient-bearing flow of water through 
them. Evaporating this water typically takes 40 percent or more of the energy that falls on a 


plant, and a much higher percentage of what is actually absorbed by the plant. (For production 

of every ton of wet weight of many crops — of which about one-fourth is dry organic matter — 
around 100 tons of water are evaporated. This means 400 grams of water are evaporated, 

Odum, Fundamentals, Chapter 3. 

D. Gates, The flow of energy in the biosphere. 


Ibid. ; H. Penman, The water cycle. 









CO* H/3 



•o y 


"""X^ R£SP«ATK3N 







FIGURE 3-2 The cycling of carbon through organism (simplified). 

about 0.6 kilocalories per gram evaporated, for every gram or 4.5 kilocalories of dry organic 
matter produced. The ratio in such a case is about 50 kilocalories for evaporation per kilocalorie 
of net primary production.) 

Oxygen balance: metabolism by animals and microbes. The overall effects of the net 
production of plants on the cycles of carbon, oxygen, and hydrogen are as follows: (1) more 
oxygen, O 2, is added to the environment than is removed from it; (2) less carbon dioxide, CO 2, 
and water, H 2 O, are added to the environment than are removed from it; (3) the CO 2 and H 2 O 
subtracted from the environmental pools show up as additions to the pool of stored organic 
matter, CH 2 0, and to the atmospheric oxygen pool. Quantitatively, for every metric ton of dry 
organic matter produced, there is a net loss from the environment of 0.6 metric tons of water and 
about 1.5 metric tons of CO 2, and a net gain of about 1.1 metric tons of oxygen. 

Why, then, is oxygen not building up continuously in the atmosphere, while water and carbon 
dioxide are depleted? Because plant matter is not accumulating, but rather is approximately in 

steady state. It is being broken down into carbon dioxide and water again by the metabolism of 
animals and microbes, using up molecular oxygen just as rapidly as new plant material and 
oxygen are being produced. 

Consider the possible fates of plant carbohydrate produced as part of net primary production ( 
Figure 3-2 ). Plants either are consumed by herbivores, or they die and add their carbohydrate to 
the pool of dead organic matter. Of the part consumed by herbivores, some is metabolized and 
thereby returned to the environmental pool of CO 2 and H 2 O, some is added as excrement to the 
pool of dead organic matter, and some is incorporated into herbivore tissue, either to be eaten by 
carnivores or to be added at the herbivore's death (by causes other than being devoured) directly 
to the pool of dead organic matter. The same fates await the carbohydrate that reaches the 
carnivores. The cycle is closed by the action of decomposers—both animals and microbes 
(bacteria)— which by metabolizing the fixed carbon in dead 


organic matter return CO and H O to the environmental pools. 

There are some important side pathways in the carbon cycle not shown in Figure 3-2 , in which 
the initial products of decomposition include methane and carbon monoxide (CH 4 and CO) in 
addition to carbon dioxide. The main reactions are: 

2 CH 2 > CH 4 + C0 2 

2 CH 2 + 2 > 2 CO + 2 H 2 

The CH 4 and CO may then be oxidized in the soil or in the atmosphere according to the 
following reactions: 

2 CH 4 + 3 2 > 2 CO + 4 H 2 


2 CO + 2 > 2 C0 2 

The details concealed by these simple formulas can be quite complex. 

In steady state, the metabolism of carbohydrate by herbivores, carnivores, and decomposers 
returns to the environment exactly as much CO 2 and H 2 O as the production of the carbohydrate 
by photosynthesis originally removed. And it consumes exactly as much oxygen as the 
photosynthesis produced. There is therefore no long-run buildup (or depletion) of oxygen in the 
environment as a result of this cycle, unless net photosynthesis and metabolism get out of 

The sequestering of fixed carbon. How did oxygen accumulate in the atmosphere in the first 
place? The biogeochemical evidence suggests that molecular oxygen in the atmosphere is of 
biological origin, there having been essentially none before the evolution of photosynthetic 


organisms, perhaps 2 billion years ago. For oxygen produced by photosynthesis to have 

accumulated over the long term, part of the carbon fixed by this process must have been 
withdrawn from the cycle illustrated in Figure 3-2 , escaping the oxidation step back to carbon 
dioxide. Indeed, it is precisely such a break in the carbon cycle that produced the fossil fuels. 
They originated as plant material that escaped breakdown by decomposers, in many cases by 
being buried in swamps and bogs under conditions in which both oxygen and anaerobic 
decomposer organisms were absent. Hundreds of millions of years of burial under considerable 
pressure and heat converted these plant materials into the hydrocarbon fossil fuels—coal, 
petroleum, and natural gas. The oxygen released when the carbon in these fuels was first fixed by 
photosynthesis remained in the atmosphere. — 

The different ranks of coal—lignite, subbituminous, bituminous, anthracite— represent different 
ages (listed here from youngest to oldest) in terms of the origin of the plant material they derived 


from, and somewhat different chemical compositions. Peat is the partially decomposed organic 

matter that is the precursor of coal. Petroleum, natural gas, oil shale, and tar sands are the results 
of different histories of biogeochemical transformation, but all originated, like coal, with dead 
organic matter. 

Not all of the fixed carbon that has been sequestered out of reach of natural oxidative processes 
is in deposits as concentrated as the fossil fuels. A much larger amount, in fact, is dispersed in 
sediments whose hydrocarbon content is on the order of 1 part per 1000 (1 g of hydrocarbon 
material per kg of sediment) or less. — 

Since the oxygen reservoir in the atmosphere has been built up by the sequestration of fixed 
carbon in various forms, could it be jeopardized by any forseeable activities of civilization that 
would cause the oxidation of these fixed-carbon pools? Deforestation, for example, could lead to 
the oxidation of part of the standing crop of fixed carbon in living organic matter, and 
combustion of fossil fuels oxidizes those pools. Yet, examination of the best estimates of the 
sizes of the fixed-carbon pools shows little cause for concern in the short term (see Table 3-6 ). 
Removal and oxidation of the entire terrestrial biomass would deplete atmospheric oxygen by at 
most 0.2 percent. Combustion of all the conventional fossil fuels (coal, petroleum, natural gas— 
of which about 90 percent is coal) thought to be recoverable ever by human effort would deplete 
atmospheric oxygen by 1.8 percent. This 

Cloud and Gibor, The oxygen cycle. 
26 Contrary to a rather widely held misconception, fossil fuels did not come mainly from 
dinosaurs or other animals. Because of the inefficiency of energy transfer up food chains and 
other factors (which are treated in Chapter 4), the rate at which fixed carbon was incorporated 
in animal tissues was much smaller than the rate at which it was incorporated into plant 


See P. Averitt, Coal; and Chapter 8 of this book. 


See, for example, T. H. McCulloh, Oil and gas. 


TABLE 3-6 Sizes of Pools of Fixed Carbon (1015 moles reduced carbon) 

Oceanic Biomass 4 - 0.1-0.6 

Terrestrial Biomass 4 - 40-70 

Dead Organic Matter, terrestrial -' - 60-750 

Dead Organic Matter, dissolved and suspended in oceans -' - 60-250 

Recoverable Coal, Petroleum, Natural Gas - 500 

Oil Shale - 24,000 

Reduced Carbon, dispersed in sediments - 1,000,000 

Molecular Oxygen, in atmosphere "' - 38,000 
a R. M. Garrels, F. T. Mackenzie, C. Hunt, Chemical cycles and the global environment. 

G. M. Woodwell and E. V. Pecan, Carbon and the biosphere. 
c M. K. Hubbert, Energy resources. 
For comparison. 

would reduce the oxygen content of the atmosphere at sea level to what it now is at an elevation 
of 150 meters. The 1975 global rate of fossil-fuel combustion would amount to a depletion of 
atmospheric oxygen by about 0.001 percent per year—far below the threshold of detectability 
with existing measurement techniques. On the other hand, the amount of reduced carbon thought 
to be present in oil shales is sufficient, if it were ever totally recovered and oxidized, to consume 
more than half the atmospheric oxygen pool. Some observers believe that most of the oil shales 
are too dispersed an energy resource ever to be economically attractive (see Chapter 8). But, in 
the event this view is not correct and civilization commences to use oil shale at a very high rate- 
say, ten times the rate at which all forms of energy were consumed worldwide in 1975— it would 
still take many centuries for the resulting depletion of atmospheric oxygen to become serious. 

The oxygen situation is made more complicated than indicated in the foregoing paragraphs by 
the existence of important pools of oxygen elsewhere than in the atmosphere. It is apparent from 
the information in Table 3-6 that other oxygen pools must exist, because the number of moles of 
oxygen in the atmosphere is much smaller than the number of moles of reduced carbon in the 
sediments and elsewhere. The oxygen produced when all that carbon was fixed and sequestered 
was tied up with the oxidation of ferrous oxide (FeO) to ferric oxide (Fe 2 O 3 ) and sulfur to 
sulfate (-SO 4) in sedimentary rocks. It may also have been tied up in the oxidation of carbon of 
inorganic origin (such as carbon monoxide from volcanoes) to carbon dioxide and carbonate (- 
CO 3 ). Although all these reactions are slow, they can account for the necessary enormous 

amounts of oxygen if they are assumed to have operated over geologic time spans. Similarly, 

alterations in conditions governing the reaction rates could lead to significant changes in 
atmospheric composition over very long periods. On shorter time scales, however, organic 
carbon pools and associated reactions are almost certainly more important, and they are unlikely 
to produce significant changes in atmospheric oxygen in less than centuries. 

Carbon dioxide. Although atmospheric oxygen depletion evidently is not a short-term threat, 
buildup of atmospheric carbon dioxide is. Every mole of oxygen removed from the atmosphere 

means a mole of carbon dioxide added. Since the number of moles of CO in the atmosphere is 
700 times smaller than the number of moles of O , a change of a specified number of moles has 
700 times the effect on the CO concentration as on the O , concentration. The combustion of 
fossil fuel that depletes oxygen by 0.001 percent per year augments the atmospheric CO by 0.7 
percent per year—not an insignificant increment. 

Not all of the added CO remains in the atmosphere, however. Some is absorbed by the oceans, 
and it is conceivable that some is being taken up in an increase in biomass. (Recall from the 
earlier discussion that the size of the biomass is not known with sufficient accuracy to 


See, for example, Cloud and Gibor, The oxygen cycle. 

detect even rather substantial changes.) An increase in the concentration of CO 2 in the 
atmosphere should increase the rate of net photosynthesis (where other nutrients are not 
limiting), and this would produce a growing biomass if the rate of oxidative metabolism by 
consumers and decomposers did not increase correspondingly. 

Accurate measurements of atmospheric CO 2 have been made since about 1957. They have been 
coupled with indirect evidence to produce a model of the rise of atmospheric CO 2 from about 
280 molecular parts per million in preindustrial times to around 325 parts per million in 1975. 
The possible climatic effects of this increase and its continuation are discussed in Chapter 11. 

An additional kind of evidence that can shed some light on the sources and fate of the CO 2 
added to the atmosphere is the abundance of radioactive carbon- 14 relative to the amount of 
nonradioactive carbon-12 in various pools. Carbon-14 is produced naturally by bombardment of 
nitrogen by cosmic rays in the stratosphere and also by civilization when nuclear bombs are 
detonated in the atmosphere; it then mixes throughout the atmosphere and is incorporated, along 
with other carbon, into plant matter. Since the half-life of carbon-14 is only 5770 years, the 

1 1 

carbon-14 buried for millions of years in fossil fuels has decayed away entirely. Thus, the CO 

2 added to the atmosphere by the combustion of fossil fuels contains no carbon-14, whereas CO 
added from pools with faster turnover does contain carbon-14. The reduction in the 14C/C ratio 
in the atmosphere because of the combustion of fossil fuels is called the Suess effect. The use of 
data on the Suess effect, combined with other information needed to help interpret it (such as the 
way the rate of natural and artificial 14C production has varied over time), has allowed 
development of the estimate that about half of the CO ever added to the atmosphere by 

combustion of fossil fuels has stayed there. The same work indicates that the land biomass 

should have increased between 1 percent and 3 percent since preindustrial times, having 
absorbed some of the industrial CO 2, while the oceans absorbed the rest. This change in land 
biomass is much too small to measure, and the exact amount is a matter of controversy. 

Global carbon cycle. Figure 3-3 p resents an internally consistent picture of the pools and flows 
of the global carbon cycle. We caution the reader once more that substantial uncertainties exist 
about many of the numbers, as noted in detail earlier, and that further research might therefore 
alter the picture considerably. In this diagram, the pools of living and dead organic matter on 

land and in the oceans are assumed to be in equilibrium. Half the CO 2 added to the atmosphere 
by combustion of fossil fuels is assumed to dissolve in the surface layer of the oceans. The fossil- 
fuel pool shown here is somewhat larger than the figure in Table 3-6 suggests, because the latter 
indicated only the part of the fossil fuels estimated to be recoverable eventually. (Some will 
remain forever in the ground, as discussed in Chapter 8.) 

Of the carbon added to the atmosphere by the respiration of animals and microbes, only a few 
percent is in the form of methane (CH 4) and carbon monoxide (CO), while the rest is carbon 

dioxide. Still, the natural sources of carbon monoxide—incomplete decomposition of organic 

matter and partial oxidation of methane in the atmosphere—exceed man-made carbon monoxide 
from fossil-fuel combustion severalfold. Methane is apparently not toxic at any concentrations 
that occur in natural environments, and carbon monoxide is toxic only at concentrations on the 
order of 100 times the global atmospheric average or more. — 


The nitrogen cycle is chemically the most intricate of the major nutrient cycles, and it is probably 
the least well understood scientifically. The several chemical forms of nitrogen to be encountered 
in this discussion are identified in Table 3-7 . The crucial role of nitrogen in all proteins makes 
this element essential to life, but most 

Lester Machta, Prediction of CO 2 in the atmosphere. 

Those unfamiliar with the basic terminology of radioactivity should look ahead to Box 8-3. 

R. Bacastow and C. D. Keeling, Atmospheric carbon dioxide and radiocarbon in the natural 

carbon cycle, pt. 2. Their actual figure for the fraction of industrial carbon dioxide remaining 

in the atmosphere between 1959 and 1969 was 49 ± 12 percent. 
33 Garrels, Mackenzie, and Hunt, Chemical cycles, p. 74. 
34 Ibid„ pp. 73-75 . 

4;ue5tB MedQ Amenta , Inc. www.qusstia.t 




; ja:en, Rest urces, Ecjyjronmen :. Ccn:-bjions: Pau! R. EhrCch - author, Anne 

Publication Information: Eock Ttlei Ecoiceica : : 

H. Ehriicti - author, John P. HflWlir 1 - autho L Publisher 

Page Number! 81. Thic i,lttft[ i BfJff JiEtf-- ct:ga ^v ^ a Pv riaht a nd. w th trie gxceptic. n c f fa ir use, nay net be further copied, d'stributed 

o r trans mitted in any form or try a 


H. Fnee^ian. Place n f Public 

:ion; San Francisco. Publication Year: 1977. 


















0* C Otnp Q t*tWn 
1 ? 








■j i 





FIGURE 3-3 Pools and flows in the global carbon cycle. Arrows denote flows in 1015 moles per 

year; boxes denote pools in 1015 moles. 
TABLE 3-7 Chemical Forms of Nitrogen 


Major nutrient form 

From NH 3 dissolved in water 

Constituent of protein 

Bulk of atmosphere 

Laughing gas, controls natural ozone cycle 

Combustion product 

Link in N cycle 

From NO oxidized in atmosphere 

Principal nutrient form 





m 3 



ra 4 + 

Ammonium ion 


m 2 + 

Amino group 


i 2 

Nitrogen gas 

i 2 

Nitrous oxide 

+ 1 


Nitric oxide 


10 2 ~ 

Nitrite ion 


10 2 

Nitrogen dioxide 


10 3 - 

Nitrate ion 


Formula Name number Comments 

Negative oxidation numbers denote more-reduced forms, and positive oxidation numbers, more- 


organisms can assimilate and use nitrogen only in specific chemical forms. Only a relatively few 
species of bacteria and algae can convert gaseous molecular nitrogen (N 2) into the more 
complex compounds that can be used by plants and animals. The principal usable forms are 
ammonia (NH 3 ) and nitrates (-NO 3 ), of which the latter is needed in greater quantity by most 
plants. The conversion of N 2 to ammonia and nitrate is called nitrogen fixation, and the species 
that can accomplish this conversion are called nitrogen-fixing organisms. Thus, although the 
atmosphere represents an enormous reservoir of molecular nitrogen, the continuation of life on 
Earth depends absolutely on the activities of these tiny, inconspicuous organisms. 

Organisms and nitrogen. The best known of the nitrogen-fixing organisms are the bacteria 

(genus Rhizobium) associated with the special nodules on roots of legumes, which are plants of 

the pea family. In a symbiotic relationship, the bacteria obtain from the plants the energy they 

need to carry out the nitrogenfixing reaction 

2N 2 + 6H 2 0^4NH 3 + 30 2 , 

and some of the ammonia, in turn, is made available to the plants for the synthesis of amino 

acids, for example, 

2NH 3 + 2H 2 O + 4CO 2 ->• 

2CH 2 NH 2 COOH+30 2 . 

(In this example the amino acid is glycine.) The capacity of such legumes as alfalfa, beans, peas, 
and clover to fix nitrogen in large quantities has led to their widespread use in crop-rotation 
schemes to replenish in the soil the nitrogen depleted by other crops. 

Although nitrogen fixation by the legume-bacteria symbiosis is thought to be responsible for 
most natural nitrogen fixation, the fixation step is also carried out by some bacteria that have 
looser associations with plants, including the free-living Azotobacter (aerobic) and Clostridium 
(anaerobic), and by various species of blue-green algae. ^_Blue-green algae are apparently 
responsible for maintaining the fertility of rice paddies in much of Asia, which are subjected to 
intensive cropping without the use of nitrogen fertilizers. 

The amino acids used by all organisms as the building blocks of protein are synthesized not only 
from ammonia but also from nitrate produced from ammonia by processes collectively called 
nitrification. The nitrification reactions yield energy, which supports the life processes of the 
nitrifying bacteria (Nitrosomonas, Nitrobacter). The actual synthesis of amino acids is carried 
out by bacteria (which differ widely in how many amino acids they can synthesize), by plants 
(which can synthesize all the amino acids needed for making proteins), and by animals (which, 
again, differ in the variety of amino acids they can produce). — 

Vertebrates—including human beings—cannot synthesize all the amino acids they need and must 
obtain intact in the food they eat the ones they cannot synthesize. The amino acids an animal 
must get from its diet are called essential amino acids; those it can synthesize are called 
nonessential. (This terminology is unfortunate, because both essential and nonessential amino 
acids are needed— the distinction concerns only whether the organism can synthesize them itself.) 

Fixed nitrogen that has been incorporated into organisms returns to the soil in animal wastes and 
in dead organisms (microbes, plants, animals) or parts of organisms (for example, leaves). 
Animal wastes are rich in urea— (NH 2)2 CO— which is the principal product of the metabolism 
of proteins in many organisms. The proteins of dead organisms are broken down into amino 
acids and other residues by bacteria and fungi of decay. The latter compounds— urea, amino 
acids, and other breakdown products of protein— are then converted into ammonia by yet another 
group of bacteria. This step is called ammonification. (The chemical reaction for the 
ammonification of an amino acid is just the opposite of aminoacid synthesis.) 

So far we have described how nitrogen is fixed by living organisms and how the fixed nitrogen is 
further processed by them. A logical question at this point is: How is nitrogen unfixed? For 
without a mechanism for 

See, for example, Odum, Fundamentals, pp. 87-91; C. C. Delwiche, The nitrogen cycle; and 
Stanier, Douderoff, and Adelberg, The microbial world. An important recent technical 
monograph is R. C. Burns and R. W. F. Hardy , Nitrogen fixation in bacteria and higher 
36 Lehninger, Biochemistry, Chapter 24, the biosynthesis of amino acids, and pp. 539-565, 
nitrogen fixation. 


Some Chemistry of Nitrogen in Organisms 




2N, + 6H,0 

2 -r ""2 

4NH, + 30- 

Energy Organism 

In Rhizobium, 



is the reverse) 


2NHj + 2H 2 + 4C0 2 ► 

2CH 2 NH 2 COOH + 50, 

2NH 4 + + 30 2 ► 

2N0 2 ~ + 4H+ + 2H 2 



Many, bacteria 
and others 








2N0 2 - + 2 
> 2N0 3 - 

4N0 3 - + 2H,0 » 

2N 2 + 50 2 + 40H- 

5S + 6KNO, + 2CaCO, — 

Energy Organism 

3K 2 SO< + 2C0 2 + 3N, 

QH 12 O ft + 6NOr * 

6C0 2 + 3H a O + 60H- + 3N 2 









Sources: Edward S. Deevey, Jr., Mineral cycles; R. M. Garrels, F. T. Mackenzie, and C. Hunt, 
Chemical cycles and the global environment; R. C. Burns and R. W. F. Hardy, Nitrogen fixation. 


k ^* 



















Aauniaftfvi m 


• ■» n ■ 











FIGURE 3-4 Biological processes in the terrestrial nitrogen cycle. 

returning nitrogen to its molecular form (N 2), even the vast reservoir of this compound that 
exists in the atmosphere would have been depleted long ago; much of the nitrogen would be tied 

up as nitrates in soil, in the oceans, and in sediments. The mechanism that has prevented this 

outcome by closing the atmospheric loop of the nitrogen cycle is the action of a relatively few 
groups of denitrifying bacteria that make their living converting nitrate to nitrous oxide (N 2 O) 
and N 2 - (These transformations release energy.) The nitrous oxide is reduced to N 2 by further 

bacterial action, or in the atmosphere by photochemical reactions. Another set of denitrifying 
reactions, also carried out by bacteria, transforms nitrate to nitrite and nitrite to ammonia. 

The chemistry of the main steps in the nitrogen cycle that are mediated by bacteria is 
summarized (in an illustrative, not exhaustive way) in Table 3-8 . For each reaction, the "energy" 
column indicates whether energy must be supplied to accomplish it ("in") or whether the reaction 
makes energy available ("out"). The part of the terrestrial nitrogen cycle directly associated with 
organisms is represented schematically in Figure 3-4 . The oceanic cycle is substantially similar, 


relying both on bacteria and algae. — 

37 See the discussion in Deevey, Mineral cycles, p. 141. 
38 See especially Burns and Hardy, Nitrogen fixation. 

Inorganic processes. Some important flows and transformations in the nitrogen cycle take place 
without the active participation of organisms. For example, some of the ammonia produced by 
the decomposition of organic materials enters the atmosphere by outgassing from Earth's surface. 
Highly soluble in water, the ammonia dissolves in atmospheric water vapor to form ammonium 
ion, which combines with sulfate and nitrate ions and rains out as ammonium sulfate and 
ammonium nitrate. Possibly, the ammonium ion is also partly removed from the atmosphere by 
oxidation to N 2. — 

Nitrous oxide, produced by denitrifying bacteria, is the second most abundant form of nitrogen in 
the atmosphere, after N 2- No sinks for this gas are known in the troposphere—that is, no process 
has been identified in the troposphere that converts N 2 O to other compounds—but one may yet 
be discovered. The presently known sink for N 2 O is in the stratosphere, where it is both 
photochemically reduced by ultraviolet light to produce N 2 and O, and oxidized by contact with 
atomic oxygen to produce nitric oxide (NO). 40 The net result chemically is 

2N 2 > N 2 + 2NO 

Nitric oxide (NO) is a very effective catalyst for the destruction of ozone (O 3 ) in the 
stratosphere. The reactions are 

NO + 3 > N0 2 + 2 

O ultraviolet v -. _ 

3 — nsr^ o 2 + o 

NO, + O > NO + O 

from which the net effect is 

or% ultraviolet . 

2 °3 ii^ 30 2 

Thus, there is a connection between the rate of biological nitrous oxide (N 2 O) production in the 
soil and the rate of ozone destruction high in the stratosphere. This link is examined more closely 
in Chapter 1 1 . 

Nitric oxide is also produced in or added to the troposphere by several processes. One is the so- 
called juvenile addition of nitric oxide by volcanoes. Another is fixation from atmospheric 
nitrogen gas by lightning, where the lightning discharge supplies the high energy needed to drive 
the reaction: 

N 2 + 2 > 2NO 

The same reaction takes place as the result of combustion of fossil fuels by industrial society in 
automobiles, aircraft, electric power plants, and other processes. Nitric oxide, from whatever 
source, is oxidized in the troposphere to give nitrogen dioxide (NO 2 ), which in turn reacts with 
atmospheric water, giving nitric acid and reproducing the NO: 

3NO, + 3H,0 > 2HNO, + NO 

The nitric acid falls in rain as HNO 3 or reacts with other atmospheric constituents (such as NH 4 
+) to produce other nitrate compounds. The end result in either case is to supply fixed nitrogen to 
the surface. 

In addition to the inadvertent production of fixed nitrogen through the combustion of fossil fuels, 
industrial society fixes atmospheric nitrogen intentionally for use as fertilizer. The basis of this 
industrial fixation process is a method invented by Haber and Bosch in 1914, in which nitrogen 
and hydrogen react under high pressure and in the presence of a catalyst to form ammonia. The 
source of the hydrogen for this process is usually methane in natural gas. Some of the ammonia 
produced is reacted with carbon dioxide to produce urea, and some is reacted with oxygen to 

form nitric acid. The nitric acid is reacted with more ammonia to make ammonium nitrate—along 

with urea, a widely used fertilizer. These materials are often referred to as inorganic fertilizers, 

to distinguish them from the ammonia, urea, and nitrates of organic origin that are often used as 
fertilizer in the form of manure or plant material. (There is no chemical distinction between, say, 
nitrate ion of organic origin and nitrate ion produced in a fertilizer plant, but important 
differences in soil quality may result from the different ways in which the nitrate is actually 
made available to the soil in organic and inorganic fertilization. See Chapter 1 1 for further 


Garrels, Mackenzie, and Hunt, Chemical cycles, p. 95. 

P. J. Crutzen, Estimates of possible variations in total ozone due to natural causes and human 

Delwiche, The nitrogen cycle, p. 143. 



FIGURE 3-5 The global nitrogen cycle. The largest flows are denoted by the heavier arrows. 

Nitrate compounds, whether put in the soil by biological processes or by rainout from the 
atmosphere or by application of inorganic fertilizers, are usually quite soluble; thus, those that 
escape uptake by plants or bacterial denitrification may be transported far and wide by the flow 
of surface water, or they may accumulate in the much more slowly moving reservoirs of 

Global nitrogen cycle. The biologic and inorganic steps in the global nitrogen cycle are depicted 
together in Figure 3-5 . Estimates of the sizes of some of the major flows are summarized in 


Table 3-9 , with an indication of the approximate uncertainties in those figure. — 

Note that industrial nitrogen fixation may have been equal to half of natural fixation already in 
1975, a very significant perturbation in a global process. It is unlikely that natural denitrification 
processes are keeping pace with the increased load of fixed nitrogen (although the data are far 
from adequate to prove this). If this is the case, fixed nitrogen must be accumulating in one or 
more of the pools: biomass, dead organic matter, inorganic pools in soil and oceans, or 
groundwater. How much increase in global river flow of nitrates has increased because of 
industrial fixation is quite uncertain—and controversial (see Chapter 11). Also very unclear is the 
partitioning between N 2 O and N 2 of gas flow to the 


The ranges of uncertainty are from the workshop summarized by the Institute of Ecology 
(TIE) in Man in the living environment, and from comparisons of some of the principal 
published reviews of nitrogen cycle data and estimates, namely Garrels, Mackenzie, and 
Hunt, Chemical cycles; chapter 8; Delwiche, The nitrogen cycle; Deevey, Mineral cycles; 
Burns and Hardy, Nitrogen fixation. A set of estimates giving natural fluxes around 10 times 
greater than the central values presented here for some fixed-nitrogen forms appears in E. 
Robinson and R. C. Robbins, Gaseous atmospheric pollutants from urban and natural sources, 
in Global effects of environmental pollution, S. F. Singer, ed. Those estimates apparently were 
not considered credible by those attending the TIE workshop. 

TABLE 3-9 Magnitudes of Flows in the Global Nitrogen Cycle 



Other Terrestrial Biological 

Oceanic Biological 

Atmospheric by Lightning 

Fixed Juvenile Addition 

Fertilizer Production 


Other Processes 

(TOTAL) - 

Terrestrial Plants 

Terrestrial Fungi and Bacteria 

Oceanic Plants 

(10 moles N/yrj 


» uncertainty 


± 50% 


+ x2 


+ x3 


+ x2 


+ x3 


± 20% 


± 10% 


± 25% 


± 50% 


+ x2 


± 50% 






-x 5 




+ x2 


+ x2 


+ x2 


+ x2 


+ x3 




+ x2 


+ x2 


+ x2 

Magnitude Approximate 
(70 moles N/yr) uncertainty 

Oceanic Fungi and Bacteria 


From Land 

From Ocean 

From Land 

From Ocean 

To Land 

To Ocean 


°The symbol ±x_2 means the value could be from 0.5 to 2 times the magnitude given. 

The figures for assimilation of fixed nitrogen by terrestrial and oceanic plants are derived from 
the net 

primary production (carbon) estimates of Table 3-4 , using the molecular carbon/nitrogen ratios 
of 41/1 for 

terrestrial plant production and 5.7/1 for oceanic plant production ( TIE, Man, p. 71). The figure 
for terrestrial 

plants differs from the molecular carbon/nitrogen ratio in the terrestrial pool of plants (about 
90/1, according to 

Deevey, Mineral cycles) because the pool is mostly wood, whereas the annual production 
contains a much higher 
proportion of leaves. 

Sources: C. C. Delwiche, The nitrogen cycle; R. M. Garrels, F. T. Mackenzie, and C. Hunt, 

Chemical cycles 

and the global environment, chapter 8. The Institute of Ecology (TIE), Man in the living 

environment, chapter 3. 

R. C. Burns and R. W. F. Hardy, Nitrogen fixation in bacteria and higher plants . 

atmosphere from denitrification, and what factors control the partitioning. — 

Perhaps the most striking feature of the data in Table 3-9 is the margin by which the rate of 
uptake of fixed nitrogen for incorporation into protein by plants, fungi, and bacteria exceeds the 
rate of new fixation by both natural and industrial processes (roughly a factor of 35). The internal 
loops of the nitrogen cycle, which can be summarized as uptake/synthesis — » excretion/death — > 
decomposition — ► uptake/synthesis (with the nitrogen remaining in the fixed state at all times), 
thus embody a great deal more of the cycle's total activity than do the external loops, wherein 
nitrogen enters and leaves the atmospheric pool of N 2 (see Figure 3-5 ) . Among other things, this 
suggests that disturbances directly affecting the strong internal loops would lead much more 

quickly to large-scale consequences than would disturbances affecting fixation of atmospheric 

Estimates of the sizes of some of the pools of nitrogen in the global cycle appear in Table 3-10 . 
A significant contrast with other biogeochemical cycles considered here is the large fraction of 
the total reservoir of nitrogen that is tied up in the atmospheric pool; for all other 

43 For a synopsis of such insights as are available, see Burns and Hardy, Nitrogen fixation. 

nutrients, the pools in sediments and sedimentary rocks are largest. 

From the information on pools in Table 3-10 and that on flows in Table 3-9 , some important 
characteristic times can be deduced, albeit with substantial uncertainties. For example, the 
assimilation of fixed nitrogen by terrestrial plants "turns over" the nitrogen in the combined dead 
organic matter and inorganic soil nitrogen pools roughly every 100 to 600 years; assimilation of 
fixed nitrogen by ocean plants turns over the corresponding ocean pools every 40 to 350 years. 
The stock of nitrogen in the main terrestrial and oceanic loops (biomass, dead organic matter, 
fixed nitogen in soil and water) is replenished by natural fixation every 1200 to 35,000 years- 
turnover times that apparently would be cut by 50 percent by the addition of the 1975 rate of 
industrial fixation. The atmospheric pool of N 2 is turned over only once every 15 million to 60 
million years by natural fixation. By contrast, N 2 O has an atmospheric residence time of 
perhaps ten to thirty years (unless its production and destruction rates have been badly 
misjudged), and NH 3 /NH 4 and NO/NO 2 have residence times of a month or two. The shortest 
residence time among atmospheric nitrogen forms is that of NO 2 -and NO 3 — perhaps a day. 

Clearly, human interventions in the nitrogen cycle cannot possibly influence the atmospheric 
concentration of N 2 on any time scale of practical interest. On the other hand, the concentrations 
of other atmospheric forms of nitrogen could be influenced on time scales of a few months to a 
few years, and the sizes of the major pools of fixed nitrogen in the soil and surface water could 
be influenced in a matter of a few to a few tens of human generations. Significant local 
disruptions in soil and surface water can of course occur more quickly, and they have (see 
Chapter 11). 


Phosphorus, while absolutely essential to life, is required in quantities only about one-tenth as 
great as nitrogen. ^Nevertheless, phosphorus probably is the 

TABLE 3-10 Sizes of Pools in the Global Nitrogen Cycle 

Pool (10 12 moles N) 


Biomass (99% in plants) - 400- 1 ,000 

Pool (10 12 moles N) 

Dead Organic Matter - 5,000-60,000 

Inorganic Fixed Nitrogen in Soil h - 5,000-15,000 


Biomass (80% in plants?) 4 - 10-300 

Dead Organic Matter a - 9,000-60,000 

Dissolved Inorganic Nitrogen in Oceans & - 4,000-50,000 

Dissolved N 2 in Oceans * - 1 ,500,000 ± 1 0% 

SEDIMENTS & - 29,000,000-70,000,00 


N 2 282,000,000 

N 2 O 90 ± 50% 

NH 3 ,NH 4 + 2 ±50% 

NO, NO 2 0.4 ± 50% 

"Based on the range of values for carbon pools in Table 3-6 , with 
carbon/nitrogen ratios as follows: land biomass, 90/1 ( Deevey, "Mineral cycles"; 
Garrels, Mackenzie, and Hunt, Chemical cycles); land, dead organic matter, 12/: 
( Woodwell and Pecan, eds., Carbon, p. 369); ocean biomass and dead organic 
matter, 6.6/1 ( Garrels, Mackenzie, and Hunt, Chemical cycles). 

1 The Institute of Ecology, Man in the living environment. 

c Burns and Hardy, Nitrogen fixation. 

Garrels, Mackenzie, and Hunt, Chemical cycles, chapter 8. 
e Delwiche, The nitrogen cycle. 

E. Almqvist, An analysis of global air pollution. 

limiting nutrient in more circumstances than any other element because of its scarcity in 
accessible form in the biosphere. 

Two chemical properties of phosphorus are responsible for this natural scarcity, which is much 
more acute than one might expect from the size of the total phosphorus pool in sedimentary 
rocks. One is that phosphorus does not form any important gaseous compounds under conditions 
encountered in the environment. ^_The second is the insolubility of the salts formed by the 
phosphate anion PO 4 = and the common cations Ca ++ , Fe ++ , and Al +++ . The lack of gaseous 
compounds deprives the phosphorus cycle of an atmospheric pathway linking land and sea and 
thus slows the closing of the cycle to the almost inconceivably sluggish pace of 

44 N/P atom ratio is 9/1 in land plants, according to Deevey, Mineral cycles, and 16/1 in marine 

plants, according to Garrels, Mackenzie, and Hunt, p. 67. 
45 Phosphene gas produced in swamps is negligible in quantity. See, for example, Institute of 

Ecology, p. 50. 







FIGURE 3-6 The global phosphorus cycle. Pools are in units of 10 moles; flows, in units of 

1 9 

10 moles per year. 

sedimentation, uplift, and weathering. That phosphate forms insoluble compounds with 
constituents of most soils retards its uptake by plants and slows its removal and transport by 
surface water and groundwater. 

The main characteristics of the global phosphorus cycle are indicated in Figure 3-6 . The numbers 
given there for the organic pools and flows are based on the carbon-cycle estimates summarized 
in Figure 3-3 , combined with the carbon/phosphorus ratios for different kinds of organic matter, 
as discussed above. These phosphorus numbers, therefore, should not be considered any more 
precise than the carbon numbers (roughly ± 50 percent in the flows, and multiplicative factors of 
2 or 3 larger or smaller in the pools). In fact, they may be less precise than this, because of 
inaccuracy in the estimates of carbon/phosphorus ratios on a global basis. 

As in the nitrogen cycle, the inner loops between living material and the soil (or ocean) pool of 
decomposition products appear to contain most of the flow in the phosphorus cycle. Also in 
analogy to the nitrogen cycle, a group of bacteria (phosphatizing bacteria) makes a living 
converting the phosphorus compounds characteristic of living tissue into inorganic phosphate. 

The links between the terrestrial and oceanic parts of the phosphorus cycle are very weak. Little 
dissolved phosphorus is carried by rivers, because of the low solubility of phosphorus salts (the 
amount carried in suspended soil particles by erosion is about ten times as large). Aside from the 
slow link provided by the sedimentary cycle, the only sea-to-land transport is in fish and shellfish 
harvested from the sea and consumed on land, and in the excrement that fish-eating seabirds 
deposit on land. These flows together amount to about one-hundredth of the erosion loss. The 
idea that the sedimentation rate roughly balances the river flow and is balanced in turn by the 
rate of uplifting and weathering is an assumption, not an experimentally determined fact. The 
assumption amounts to supposing that the 


various pools have reached a natural equilibrium over geologic time and that the extra 
phosphorus now being mobilized by civilization's mining of phosphate rock is not yet reaching 
the rivers in appreciable quantities. 46 Quite possibly, the acquisition of more data shedding light 
on historical and contemporary phosphorus loads in rivers will change this picture. 

Worldwide mining of phosphate rock amounted to 94 million metric tons in 1972, which 
corresponds to the 0.6 trillion moles of phosphorus represented under "mining" in Figure 3-6 . — 
Seventy to 80 percent of this amount is added to the land as fertilizer ~ mostly in the forms of 
ammonium phosphate [(NH 4 ) 3 PO 4 ], triple phosphate [Ca(H 2 PO 4) 2], and superphosphate, 
which is a mixture of triple phosphate and gypsum [CaSO 4 '2H 2 O]. ^.Phosphorus is also used 
in detergents (three-quarters of the nonfertilizer use), animal-feed supplements, pesticides, 
medicines, and a host of industrial applications. ^_It seems reasonable to assume that most of 
these uses eventually lead to the deposition of the phosphorus in the environment. 

Over the long term, civilization's mobilization of phosphorus by mining phosphate rock must 
certainly be considered a significant perturbation in the cycle. The time in which mining at the 
level of the mid-1970s would double the pool of phosphate in the soil (assuming the additions all 
remained there, which is unlikely) would be around 10,000 years. There are some reasons to 
think that this very long-term problem should not be our primary worry about the phosphorus 
cycle, however. One is that the local perturbation of adding phosphorus to agricultural land is far 
greater than these global-average numbers indicate and can be important much sooner (see 
Chapter 1 1). A second is that known and suspected minable phosphorus resources — as 
distinguished from those too dispersed to recover — could not support a drain of 0.6 trillion 
moles of phosphorus per year for 10,000 years (see Chapter 9); this situation suggests, especially 
in the face of a demand for phosphorus that is not constant but, rather, is doubling about every 
fifteen years, that exhaustion of the concentrated supplies of phosphorus is more imminent than 
overloading the cycle globally. These aspects of civilization's effect on the phosphorus cycle are, 
of course, closely related. It is largely because society breaks the main internal loop of the 
phosphorus cycle on agricultural land (by "exporting" from the land the phosphorus-containing 
crops and crop residues and not returning the sewage that in time receives this phosphorus) that 
the heavy supplement of mined phosphorus is required. 


The sulfur cycle is important, chemically complicated, and not yet well understood 
quantitatively. Its importance has several dimensions: the essential role of sulfur in the structure 
of proteins; the circumstance that the main gaseous compounds of sulfur are toxic to mammals; 
the fact that sulfur compounds are important determinants of the acidity of rainfall, surface 
water, and soil; and the possibility that sulfur compounds may play a role in influencing the 
amount of molecular oxygen in the atmosphere in the very long term. ^The complexity of the 
sulfur cycle, like that of nitrogen, arises mainly from the large number of oxidation states the 
element can assume. Some of the principal compounds and groups in which sulfur participates 
are listed in Table 3-11 . 

Many transformations among the different oxidation states of sulfur are carried out by bacteria. 
Which of the several kinds of sulfur bacteria (and, hence, which reactions) prevail in a given 
situation depends on the presence or absence of oxygen and light and the acidity or alkalinity of 
the environment. 51 The principal biologic sulfur transformations are summarized in Table 3-12 . 
Essentially all of the bacterial activity probably takes place in wet media ~ moist soil, swamps, 
the muds of lakeshores and estuaries, and (to an uncertain degree) the 

46 See, for example, A. Lerman, F. T. Mackenzie, and R. M. Garrels, Modeling of geo chemical 

U.S. Department of Commerce, Statistical abstract of the United States, 197 '4 p. 675. 
Phosphate rock is calcium phosphate, Ca 3 (PO 4 ) 2 , which is 20 percent phosphorus by 
Raymond Ewell, Fertilizer use throughout the world; J. B. Cathcart and R. A. Gulbrandsen, 

49 c 

Phosphate deposits. 

Study of Critical Environmental Problems, Man's impact on the global environment. 
3 A thorough and i 
the environment. 
^ee, for example 

50 A thorough and up-to-date review of the sulfur cycle is Missouri Botanical Garden, Sulfur in 
See, for example, H. D. Peck, Jr., The microbial sulfur cycle. 

TABLE 3-11 Chemical Forms of Sulfur 

Formula Name Oxidation number Comments 

"Rotten egg" gas, extremely toxic 
Constituent of amino acids 
Forms insoluble compounds with metals 
Plays crucial role in stiffening protein 
Crystalline solid 
Colorless, toxic gas 
Weak acid from SO 2 plus water 
Gas from oxidizing SO 2 in air 
Strong acid from SO 3 plus water 
Forms many compounds in atmosphere and soil 
TABLE 3-12 Sulfur Chemistry of Biologic Assimilation and Decomposition 

H 2 S 

Hydrogen sulfide 



Hydrosulfide ion 



Sulfide ion 


s 2 = 

Disulfide ion 


S 2>S 6,S 

s Elemental sulfur 

SO 2 

Sulfur dioxide 



Sulfurous acid 


SO 3 

Sulfur trioxide 



Sulfuric acid 


so 4 = 

Sulfate ion 


Transformation Mechanism 

Assimilation and synthesis by plants 

S0 2 , S0 4 - * 

organic S 

Organic S 

*H 2 S 

Many anaerobic and aerobic bacteria 

Organic S 

>so 4 = 

— > 

Most plants and animals, many bacteria 

so 4 = 


Anaerobic bacteria (Desulforvibrio, Desulfotomaculum) 



2 "" Aerobic bacteria (Thiobacillus), photosynthetic 

O /™\ — bacteria (Chromatium, Chlorobium) 

— >so 4 - 

open water of lakes and oceans. The presence of dissolved oxygen favors decomposition of 

organic sulfur to form sulfates; the absence of oxygen favors decomposition to form sulfides. 

Reactions involving the oxidation of sulfur also take place without the intervention of bacteria. 
Hydrogen sulfide that has been produced by bacterial decomposition is readily oxidized 
chemically by dissolved oxygen in water, producing sulfite and sulfate. (The mean lifetime of H 
2 S in water containing appreciable dissolved oxygen appears to be on the order of tens of 
minutes.) ^Hydrogen sulfide emitted to the atmosphere is oxidized to sulfur dioxide (SO 2) by 
atomic oxygen (O), molecular oxygen (O 2 ), and ozone (O 3 ). These reactions are too slow in 
clean, dry air to account for the observed atmospheric residence time of H 2 S of only a few 
hours; possibly most of the action takes place when the reacting gases are dissolved in 
atmospheric water droplets or absorbed on the surfaces of suspended particles. ^Sulfur dioxide 
is further oxidized in the atmosphere to form sulfur trioxide and various sulfates, including 
sulfuric acid. Photochemical oxidation of SO 2 exists but is too slow (0.1 percent per hour) to 
account for the observed lifetime of SO 2 in the atmosphere (minutes to days). Much more 
important, apparently, is the dissolving of SO 2 in atmospheric cloud and water droplets to form 
sulfurous acid (H 2 SO 3 ), followed by the oxidation of this sulfurous acid to sulfuric acid (H 2 

SO 4 ). The oxidation is sped up by various metal salts dissolved in the droplets, which serve as 
catalysts. — 

The global sulfur cycle is represented in schematic and simplified form in Figure 3-7 . The 
estimates given there 


F. B. Hill, Atmospheric sulfur and its links to the biota; A. R. Brigham and A. U. Brigham, 

Sulfur in the aquatic ecosystem. 

H. G. Ostlund and G. Alexander, "Oxidation rate of sulfide in seawater: A preliminary study" 

Journal of Geophysical Research, vol. 68 ( 1963), pp. 3995-3997. 
54 See R. D. Cadle, The sulfur cycle, in Sulfur, Missouri Botanical Garden; W. Kellogg, R. 

Cadle, E. Allen, A. Lazrus, E. Martell, The sulfur cycle. 

P. P. Gaspar, Sulfur in the atmosphere, Sulfur, Missouri Botanical Garden, pp. 14-38. 












SO, so, 


ft* Burning 


H*n. ttfoul 

SO,, SO, 







animai *] 









Rain, lattout 

SO,. SO, 



1 3 

• I 

'• s 






ndvrtinaJ SO* 


1 1 F*Sj 
8 CaSO, 





■-*-*m"^^"i *i« ^ *-Ji-Jm^ 






so. t 



300 mmo* C«SO, 
1»m*or> F»S, 




FIGURE 3-7 The global sulfur cycle. Pools are in 
units of 10 12 moles of sulfur; flows, 

1 ? 

in units of 10 moles per year. 

for the magnitudes of pools and flows contain all the uncertainties encountered in the carbon, 
nitrogen, and phosphorus cycles, and more. The pools in organic matter were computed from the 
carbon estimates given in Figure 3-3 , together with carbon/sulfur atom ratios of 800/1 in living 
terrestrial plants and 106/1 in marine plants and dead organic matter. 56 Uptakes were computed 
from the phosphorus figures in Figure 3-6 (in turn, based on carbon) with phosphorus/sulfur 

atom ratios of 1/1 for land plants and 2/1 for ocean plants. The global average sulfur content of 

organic matter is not considered as well established as the carbon, nitrogen, and phosphorus 
contents, however. 

The values for the flows to the atmosphere of SO 2 and H2S originating in bacterial 
decomposition were chosen in Figure 3-7 to produce a balance between inputs and outputs of 
sulfur to the atmosphere; there are at this writing no adequate data with which to support or 
reject those values for the SO 2 and H 2 S flows, nor are measurements of the atmospheric pools 
of H 2 S, SO 2, and SO 4 = accurate enough to determine whether they are really constant at 
present or not. The apparent mean lifetime of sulfur in all forms in the atmosphere (about 10 11 
moles of sulfur altogether) is about one week. The main pathways by which sulfur leaves the 
atmosphere are dry fallout and rainout of SO 2 and sulfates [the latter largely as H 2 SO 4 and 
neutral ammonium sulfate, (NH 4 ) 2 SO 4 ] and uptake of SO 2 and sulfates directly from the 


atmosphere by plants. — 

Ratios are from Garrels, Mackenzie, and Hunt, Chemical cycles, p. 67. 

The land-plant ratio is based on date for uptake by crops in M. B. Jones , Sulfur in agricultural 
lands, in Sulfur, Missouri Botanical Garden, pp. 141-168; the ratio for ocean plants, from 
Garrels, Mackenzie, and Hunt, Chemical cycles, p. 67, assuming high turnover tends to make 
P/S ratio the same in uptake as in the pool. 
58 See, for example, J. G. Severson, Jr., Sulfur and higher plants, in Sulfur, Missouri Botanical 
Garden, pp. 92-111. 

Sulfates and sulfuric acid that fall on the land can usefully restore nutrient sulfur that has been 
removed by cropping, or they can acidify soil and surface water with possible adverse effects on 
ecosystems, depending on the circumstances. The human input of sulfur to the atmosphere had 
reached a magnitude of about half of the natural inputs by the early 1970s (if the estimates of the 
natural flows in Figure 3-7 are roughly correct) and had apparently caused an increase in the 
acidity of rainfall over large regions (see Chapter 11). The human perturbation in the sulfur cycle 
appears all the more significant when the additions to land and surface water of sulfur fertilizer 
and industrial sulfuric acid are reckoned in, since they amount to about half as much as the 
human sulfur inputs to the atmosphere. Figure 3-7 indicates an annual excess of 0.8 x 10 moles 
of sulfur accumulating in the soil or terrestrial vegetation and leaves open the possibility that the 
other 1.6 x 10 moles of the annual human sulfur input are accumulating in the oceans. Again, 
these are little more than educated guesses; knowledge of the magnitude of the relevant pools 

and flows is inadequate to determine what really is happening in this much detail. We simply 
know the extra sulfur must go somewhere. 

Potentially capable of altering the atmospheric oxygen pool over the very long term ~ tens to 
hundreds of millions of years — is the balance or imbalance between weathering and 
sedimentation of highly insoluble iron sulfide (FeS 2)- Weathering of FeS2 produces sulfate, 
which under some circumstances can be precipated into sediments as gypsum (CaSO 4 ■ H 2 O) 
before being reduced back to sulfide. The precipitation of gypsum, of course, takes the oxygen in 
the sulfate (which was extracted from the atmosphere during the weathering of the FeS 2) with it. 
If more sulfur is added to sediments as gypsum than is removed from sedimentary rocks as 
gypsum — that is, if there is an increase of sedimentary CaSO 4 H 2 O at the expense of FeS 2 — 
then a steady drain on atmospheric oxygen could result. There is evidence that this phenomenon 
was actually occurring during the Permian period about 250 million years ago, but how much 
oxygen depletion occurred is unclear and controversial. — 

One rather powerful analytic technique offers hope of resolving some of the important 
uncertainties about the sulfur cycle, in both its short-term and long-term aspects. The technique 
exploits the existence of two stable isotopes of sulfur — S and S — the relative proportions of 
which differ in sulfur compounds of different origins. The average abundance ratio of S atoms 
to 34S atoms in nature is thought to be about 22.2/1, but sulfur in seawater has 2 percent more 


S than this, biologically produced hydrogen sulfide has between 2.3 percent less and 0.6 
percent more, and so on. ^_Since instruments are now available that can measure this ratio with 
great precision, in a sample it is possible to determine, for instance, the fraction of a given flow 
of sulfur that comes from different pools (if they have different ratios). This technique has been 
used successfully to determine the origins of pollutant sulfur in several places around the world, 
and to investigate the fate of sulfur in the sedimentary cycle in earlier geologic eras. 
Discrimination on the basis of isotope ratios also has great potential for untangling some of the 
complexities of the carbon, nitrogen, phosphorous, and other cycles. 


The nutrients just considered in detail are, of course, not the only important ones, nor are the 
quantitative relationships shown for global averages uniformly valid in different kinds of 
biological communities (or even in the same general kinds of communities in different places). 
We chose carbon-oxygen-hydrogen, nitrogen, phosphorus, and sulfur either because of their 
quantitative importance globally (C-O-H and N) or because of special characteristics of their 
biochemical roles and environmental behavior (P and S), and because a larger literature exists on 
these nutrients than on the others. 

Other nutrients mobilized and used in large quantities include calcium, magnesium, and 
potassium. All of these are present in seawater in large amounts (see Table 2-7 ) and in rainwater 
and surface water in much smaller 

59 Garrels, Mackenzie, and Hunt, Chemical cycles, pp. 87-89. 

60 For a thorough discussion, see B. D. Holt, "Determination of stable sulfur isotope ratios in the 

environment", Progress in nuclear energy, analytical chemistry, vol. 12, no. 1 ( 1975), pp. 11- 

amounts, and are added to the soil by rock weathering as described above in the discussion of the 
sedimentary cycle. In some circumstances, dust raised by the wind and deposited as dry fallout 
elsewhere forms an important additional link in the cycling of these nutrients, and in other 
circumstances the mobilization rate from rocks is increased by the rock-splitting action of deep 
tree roots in the weathering process. — 

The cycling of the macronutrients nitrogen, phosphorus, sulfur, calcium (Ca), magnesium (Mg) 
and potassium (K), in various kinds of biological communities -hardwood forests, jungles, 
deserts — has been extensively studied since the 1960s, and a large body of data is accumulating. 
2_The enormous variety in the nutrient budgets of different communities in different places is 
indicated graphically in Figure 3-8 , which summarizes some of these studies. The differences 
among communities of the same type (for example, the temperate hardwood forests) result from 
differences in soil, quantity of rainfall, local rainwater chemistry, dry fallout characteristics, and 
detailed species composition. (All these factors, of course, interact.) 

One respect in which the presentation of global nutrient flows may be particularly misleading is 
in failing to reveal what part of the soil pool of nutrients is actually accessible to plants. This 
factor varies widely from one region to another, but it is known that in many important 
biological communities, such as tropical jungles, the actual availability of soil nutrients is very 
low. Such a cycle, in which nutrients spend a long time in a large pool of living and dead organic 
matter and a short time in a small pool of accessible inorganic nutrient forms, is called a tight 
cycle. The opposite situation, with a large pool of accesssible inorganic nutrient forms compared 
to the organic pool, is called a loose cycle. — 

Another important distinction left out of the global estimates is between agricultural and 
nonagri cultural systems. Although the human inputs of nitrogen, phosphorus, and sulfur 
fertilizers are already noticeable perturbations in these cycles on a global basis, their effects in 
the agricultural systems where they are applied are much larger than the global figures indicate. 
Much more extensive local studies are needed to resolve the questions that these magnified local 
perturbations raise (see Chapter 11). 

Potatoes '^^i^^HP^^^ Wheat 

Sugar beets 



FIGURE 3-8 Graphic representation of different nutrient budgets. The shaded polygons are 

formed by plotting on the axes shown the annual absorption of potassium, calcium, magnesium, 

nitrogen, phosphorus, and sulfur and then connecting the resulting points with straight lines. 

(Condensed from Duvigneaud and Denaeyer-de-Smet, 1975, p. 141.) 

See for example, Whittaker, Communities, chapter 5. 

2 A classic paper is F. H. Bormann and G. E. Likens, Nutrient cycling. For recent reviews, see 
D. E. Reichle, Advances in ecosystem analysis; P. Duvigneaud and S. Denaeyer-de Smet, 
Mineral cycling in terrestrial ecosystems. 
See Whittaker, Communities . 

Recommended for Further Reading 

Deevey, Edward S., Jr. 1970. Mineral cycles. Scientific American, September, pp. 148-158. A 
basic reference on nutrient cycles by a senior researcher in the field. 

Ehrlich, Paul; R. Holm; and M. Soule. 1973. Introductory biology. McGraw-Hill, New York. 
Readable introduction to the roles of elements in living systems, among many other topics. 

Garrels, R. M.; F. T. Mackenzie; C. Hunt. 1975. Chemical cycles and the global environment. 
Kaufmann, Los Altos, Calif. A good introduction to the use of simple quantitative models in the 
study of global chemical cycles. Indispensable for the serious student of these matters. 

Stryer, Lubert. 1975. Biochemistry. W. H. Freeman and Company, San Francisco. Excellent text. 

U.S. National Committee for the International Biological Program. 1975. Productivity of world 
ecosystems. National Academy of Sciences, Washington, D.C. Indispensible reference on the 
carbon cycle. 

Woodwell, G. M., and E. V. Pecan, eds. 1973. Carbon and the biosphere. National Technical 
Information Service, Springfield, Va. August, CONF-720510. Proceedings of a major conference 
with papers by most of the prominent United States researchers on the carbon cycle, global 
primary productivity, and related topics. 

Additional References 

Almqvist, E. 1974. "An analysis of global air pollution". Ambio, vol. 3, no. 5, pp. 161-167. A 
systematic survey of the magnitude of human contributions and the techniques for measuring 

Averitt, P. 1973. Coal. In D. A. Brobst and W. P. Pratt, eds. United States mineral resources, 
Government Printing Office, Washington, D. C, pp. 133-142. A technical discussion of the 
geology and physical properties of coal, as well as the magnitude of resources. 

Bacastow, R., and CD. Keeling. 1973. Atmospheric carbon dioxide and radiocarbon in the 
natural carbon cycle. Pt. 2. In Carbon and the biosphere, G. M. Woodwell and E. V. Pecan, eds., 
pp. 86-136. Excellent technical review of a complex and often less cogently presented topic — 
where the carbon dioxide comes from and where it goes. 

Bolin, B. 1970. The carbon cycle. Scientific America, September, pp. 124-132. Readable 
introduction by an eminent researcher. 

Bormann, F. H., and G. E. Likens. 1967. Nutrient cycling. Science, vol. 155, pp. 424-429. A 
good discussion by two eminent investigators of nutrient flows in terrestrial ecosystems. 

Bowen, H. J. M. 1966. Trace elements in biochemistry. Academic Press, New York. A classic 

Brigham, A. R., and A. U. Brigham. 1975. Sulfur in the aquatic ecosystem. In Sulfur in the 
environment, Missouri Botanical Garden, pp. 159-175. Good quantitative survey of the pathways 
and chemistry of sulfur in aquatic environments. 

Burns, R. C, and R. W. F. Hardy. 1975. Nitrogen fixation in bacteria and higher plants . 
Springer-Verlag, New York. Highly technical, upto-date monograph, with an extensive 
bibliography. Contains substantially higher estimate of global nitrogen fixation than previous 

Cathcart, J. B., and R. A. Gulbrandsen. 1973. Phosphate deposits. In United States mineral 
resources, D. A. Brobst and W. P. Pratt, eds. Government Printing Office, Washington, D.C., pp. 
515-525. Basic reference on the geology of the phosphate resource. 

Cloud, P., and A. Gibor. 1970. The oxygen cycle. Scientific American, September, pp. 1 1 1-123. 
Excellent introduction to the origins and fate of atmospheric oxygen on geologic time scales. 

Crutzen, P. J. 1974. "Estimates of possible variations in total ozone due to natural causes and 
human activities". Ambio, vol. 3, no. 6, pp. 201-210. A good survey of the threats to ozone, 
heavy on atmospheric chemistry. 

Delwiche, C. C. 1970. The nitrogen cycle. Scientific American, September, pp. 137-146. Good 
starting point for anyone wishing to review the recent literature of the nitrogen cycle. 

Duvigneaud, P., and S. Denaeyer- de Smet. 1975. Mineral cycling in terrestrial ecosystems. In U. 
S. National Committee for the International Biological Program, Productivity of world 
ecosystems, pp. 133-154. Strongly quantitative approach with emphasis on differences in nutrient 
flows and inventories in various ecosystems. 


Ehrlich, Paul R., and L. C. Birch. 1967. The "balance of nature" and population growth. 
American Naturalist, vol. 101, pp. 97-107. Discusses the role of herbivores in limiting plant 

Ewell, Raymond. 1972. Fertilizer use throughout the world. Chemtech, September, pp. 570-575. 
Good compilation of historical data on fertilizer use. 

Frieden, E. 1972. The chemical elements of life. Scientific American, July, pp. 52-60. One of the 
best article-length introductions to the role of chemical elements in organisms. 

Gates, D. 1971. The flow of energy in the biosphere. Scientific American, September, pp. 89- 
100. An eminent physicist-turned-biologist surveys his specialty. 

Hill, F. B. 1973. Atmospheric sulfur and its links to the biota. In Carbon and the biosphere, G. 
M. Woodwell and E. V. Pecan, eds., pp. 159-180. Good treatment, for the serious student of the 
sulfur cycle. 

Hubbert, M. K. 1969. Energy resources. In Resources and man, National Academy of Sciences- 
National Research Council, W. H. Freeman and Company, San Francisco. A classic survey and 
introduction to quantitative estimation of resource depletion by the most wellknown practitioner 
of this field. 

Ingestad, T. 1974. "Nutrient requirements and fertilization of plants". Ambio, vol. 3, no. 2, pp. 
49-54. Discusses inefficient use of nutrients under present fertilization practices and suggests 

Institute of Ecology, The (TIE). 1972. Man in the living environment. University of Wisconsin 
Press, Madison, chapter 3. Collects and summarizes the work of many authors on nutrient 

Kellogg, W.; R. Cadle; E. Allen; A. Lazrus; and E. Martell. 1972. The sulfur cycle. Science, vol. 
175, pp. 1587-1596 (February 1 1). Good, quantitative review article. 

Lehninger, A. L. 1965. Bioenergetics. Benjamin, New York. Lucid treatment of energy in 

1970. Biochemistry. Worth, New York. A thorough introduction to the chemistry of life. 

Sometimes heavy going for the uninitiated. 

Lerman, A; F. T. Mackenzie; and R. M. Garrels. 1975. Modeling of geochemical cycles. 
Geological Society of America Memoir, vol. 142, pp. 205-218. Concise presentation of some of 
the methods in the Garrels, Mackenzie, and Hunt book listed earlier. 

Levine, R. P. 1969. The mechanism of photosynthesis. Scientific American, December, pp. 58- 

McCulloh, T. H. 1973. Oil and gas. United States mineral resources, D. A. Brobst and W. P. 
Pratt, eds., pp. 477-496. Concise, fairly technical treatment of origins and occurrence of liquid 
and gaseous hydrocarbons. 

Machta, Lester. 1973. Prediction of CO 2 in the atmosphere. In Carbon in the biosphere, G. M. 
Woodwell and E. V. Pecan, pp. 21-30. Use of computer models for predicting the accumulation 
and consequences of carbon dioxide in the atmosphere. 

Maclntyre, Ferren. 1970. Why the sea is salt. Scientific American, November, pp. 104-1 15. A 
splendid introduction to geochemistry, much broader than the title implies. 

Margulis, L., and J. Lovelock. 1976. "Is Mars a spaceship, too?" Natural History, vol. 85, no. 6, 
pp. 86-90. An informative discussion of ways in which biological processes maintain conditions 
of temperature, humidity, pH, and chemical composition on and near Earth's surface. 

Mason, Brian. 1966. Principles of geochemistry. 3d ed. Wiley, New York. Good introductory 

Missouri Botanical Garden. 1975. Sulfur in the environment. Saint Louis. Best available volume 
on the chemistry and biology of sulfur compounds in the environment. Brings together much 
material that is hard to find elsewhere. 

Odum, E. P. 1971. Fundamentals of ecology. 3d ed. Saunders, Philadelphia, chapter 5. A well- 
known text, rich in field data and examples. 

Pauling, L. 1970. General chemistry. 3d ed. W. H. Freeman and Company, San Francisco. 
Excellent basic reference. 

Peck, H. D., Jr. 1975. The microbial sulfur cycle. In Sulfur in the environment, Missouri 
Botanical Garden, pp. 62-78. The often underemphasized bacterial side of this crucial nutrient 

Penman, H. 1970. The water cycle. Scientific American, September, pp. 99-108. Readable 
introduction to the hydrologic cycle. 

Press, F., and R. Siever. 1974. Earth. W. H. Freeman and Company, San Francisco. 
Magnificently illustrated, authoritative, and well written. The best geology book available, we 

Reichle, D. E. 1975. "Advances in ecosystem analysis". BioScience, vol. 25, no. 4 (April pp. 
257-264. Good survey of the nutrient-flow measurements and measurement techniques 
employed in the International Biological Program (IBP). 

Rodin, L., N. Bazilevich, and N. Rozov. 1975. Productivity of the world's main ecosystem. In 
United States National Committee for IBP, Productivity of world ecosystems, pp. 13-26. One of 
the basic papers on global primary production. Indispensable for serious students of the carbon 

SCEP. See Study of Critical Environmental Problems. 

Singer, S. F., ed. 1970. Global effects of environmental pollution. Springer-Verlag, New York. A 
useful, although uneven, collection of papers on climate, nutrients, and persistent pollutants. 

Study of Critical Environmental Problems (SCEP). 1970. Man's impact on the global 
environment. The M.I.T. Press, Cambridge. Remains a must as a survey of the human impact on 
climate and ecosystems, emphasizing effects of industrial society. Relevant to this chapter are 
treatments of carbon dioxide, phosphorus, and nitrogen. 

Study of Man's Impact on Climate (SMIC). 1971. Inadvertent climate modification. The M.I.T. 
Press, Cambridge. Contains detailed discussion of carbon dioxide and its influence on climate, 
among many other topics. 

TIE. See Institute of Ecology, The. 

United States Department of Commerce. Annual. Statistical abstract of the United States. 
Government Printing Office, Washington, D.C. 

Watson, J. D. 1976. The molecular biology of the gene. 3d ed. Benjamin, New York. The 
definitive treatise. 

Whittaker, R. H. 1970. Communities and ecosystems . Macmillan, New York. Brief, well-written 
introduction to ecology, with good treatment of global productivity. 


[This page intentionally left blank.] 


Populations and Ecosystems 

One of the penalties of an ecological education is that one lives alone in a world of wounds. 
Much of the damage inflicted on land is quite invisible to laymen. An ecologist must either 
harden his shell and make believe that the consequences of science are none of his business, or 

he must be the doctor who sees the marks of death in a community that believes itself well and 
does not want to be told otherwise. 

— Aldo Leopold, Round River 

Humanity shares the physical vehicle of Earth with an enormous diversity of other living things- 
plants, animals, and microorganisms. There may be as many as 10 million different kinds— 
species— of organisms alive today. In turn, each species consists of one or more populations, a 
population being a group of individuals more or less isolated from other populations of the same 
species. _J_This diverse array is not static. Populations of organisms change continually in size 
and genetic composition in response to changes in their physical environments and in response to 
changes in other populations. The populations of plants, animals, and microorganisms of Earth, 
or of any area of Earth, make up a biological community, a community bound together by an 
intricate web of relationships. Of course, this living web is embedded in the physical 
environment, interacts with it, and modifies it. 

The interdependence that characterizes the physical and biological elements of the environment 
has led ecologists to coin the term ecosystem (short for ecological system) for the functional unit 
that includes both biotic (living) and abiotic (nonliving) elements. In order to understand 
humanity's role in ecosystems, it is necessary 

! The reader interested in some of the complexities of the definitions of population and species 
should refer to Paul R. Ehrlich, R. W. Holm, and D. R. Parnell , The process of evolution, 
chapters 5 and 1 3 . 

to have some grasp of the functioning of both biotic and abiotic elements in these systems and of 
their properties in combination. You have already been introduced to the major abiotic features 
of the environment. In this chapter the biotic elements are discussed, starting with the properties 
of populations and then moving on to the integration of populations into ecosystems. 

The study of changes in population size-population dynamics— is of enormous practical 
importance to humankind. People are continually in the position of wishing to predict or 
influence the sizes of populations of other organisms or of some group of Homo sapiens or of 
humanity as a whole. Will there be a spruce budworm outbreak next year? Are peregrines 
threatened with extinction? Can the present harvest of sei whales or Peruvian anchovetas be 
sustained? How many people will be living in the United States in the year 2000? Answering any 
of these questions would require some knowledge of population dynamics. Although the details 
of population dynamics can be endlessly complex, the essence of analyzing changes of numbers 
is relatively straightforward. A number of points should be kept clearly in mind throughout the 

The analysis basically amounts to keeping track of inputs and outputs in a population (Box 


The inputs are natality and immigration. (Natality is used instead of births because 

individuals that hatch from eggs or grow from seeds are not normally said to have been 


The outputs are mortality (deaths) and emigration. 

In most discussions in this chapter, migration is ignored, and the analyses focus on natality 

and mortality. 

In populations with overlapping generations (such as human populations), the age 

composition— that is, the proportion of individuals in various age classes—has a substantial 

effect on the future course of population growth. In populations where adults of one 

generation invariably die before their offspring mature (like many insects), the generations 

do not overlap, and the analyses are greatly simplified. 

Although the mathematics of human population dynamics is identical to that of other animal 
populations with overlapping generations, _a subtle difference in approach has developed 
between ecologists, who are mainly interested in nonhuman populations, and the social scientists 
known as demographers, who study the dynamics of human populations alone. 

Demographers concern themselves primarily with factors that influence natality—that is, the birth 
rate. They assume that death rates in Homo sapiens will reach a low point and remain there as 
diseases and other threats are conquered, and they therefore concentrate on the interesting and 
important questions of how and when birth rates may change in response to social or 
environmental pressures. 

In contrast, ecologists— especially those who work with invertebrates— tend to focus on mortality. 
The reproductive rates of most nonhuman organisms can change dramatically only over 
evolutionary time (hundreds or thousands of generations). Ecologists have considered 
populations to have a rather fixed reproductive potential (a term no longer in vogue) and have 
viewed varying mortality rates as the prime determinants of population size. To oversimplify, 
demographers have viewed the populations they study as having varying inputs and fixed 
outputs, whereas ecologists have viewed theirs as having fixed inputs and varying outputs. The 
differences of approach in the two scientific disciplines are quite interesting and can be seen by 
comparing this discussion with that in Chapter 5, which deals with demography. 

In the remainder of this section of text, the discussion centers primarily on the dynamics of 
nonhuman populations, although most of the points apply to human beings as well. The 
treatment varies considerably in depth and complexity. The most basic concepts of population 
dynamics— for example, instantaneous birth and death rates, exponential growth, growth with 
restraint, age composition, population structure-are introduced at a 


For a more thorough introduction to the mathematics of population dynamics, see: C. J. 
Krebs, Ecology: The experimental analysis of distribution and abundance; R. W. Poole, An 
introduction to quantitative ecology; R. E. Ricklefs, Ecology; J. Roughgarden, Theory of 
population genetics and evolutionary ecology, an introduction; and E. O. Wilson and W. H. 
Bossert, A primer of population biology, Sinauer, Stamford, Conn., 1971. Only a brief outline 
is given here. 

BOX 4-1 Symbols Used in Population Dynamics 

N population size 

N(t) population size at time t. Some texts use N t . 

N(0) population size at time zero (starting population size)~sometimes written N o. 

/ time in any units 

T length of a generation 


instantaneous rate of change of population size 

IRI instantaneous rate of increase in population size 

b birth rate 

d death rate 

r intrinsic rate of increase (instantaneous rate of increase per individual) 

r m maximum intrinsic rate of increase 

A, lambda, the finite rate of increase, N i IN o 

e the base of natural logarithms (approximately 2.718) 

In natural logarithm (/og natural) 

K carrying capacity (the maximum number of individuals that can be supported in a given 

c a constant 

a alpha, a competition coefficient 

(3 beta, a competition coefficient 

x age as a variable 

Ix the proportion or number of survivors of a birth cohort at age x. 

mx the average number of female offspring produced by a female of age x 
S sum of . . . 


sum from zero to infinity of . 
R o net reproductive rate, 

NRR net reproductive rate 

n q x in life tables, the proportion of persons alive at the beginning of an age interval who die 
during that interval 

n d x m life tables, the number of persons from a given birth cohort dying during an age interval 

„L x in life tables, the number of persons in an age interval in a hypothetical stationary population 
supported by 1 00,000 annual births and subject to the actual / x schedule of the population 

T x in life tables, the total number of persons in the stationary population in an age interval and all 
subsequent age intervals. Equals 

y°° L 

Ltx n x 

level that assumes no prior familiarity with the field. For the more ambitious reader, however, 
there are also some excursions into subtleties and details at a somewhat more mathematical level. 
This material (much of it displayed in boxes) can be skipped without loss of continuity but will 
repay the scrutiny of those who wish a clearer picture of what the study of population dynamics 
really entails. The notation used in this discussion is summarized in Box 4-1. 

Instantaneous Rate of Increase 

A logical beginning is to consider the problem of measuring the rate of change in a population— 
that is, the rate of increase or decrease—a/ a given moment. An average rate is easy to calculate. 
Suppose at the beginning of a year a population of mice contains 1 00 individuals and at the end 
of the year it contains 220. The population has grown at an average rate of 10 individuals per 

month, or 120 per year. This is analogous to, say, calculating the average rate of speed of an 
automobile trip by dividing the distance between start and finish by the time it consumes. Thus, 
if the car progresses 200 miles in 10 hours, the average rate of speed is 20 miles per hour (note 
that rates are always expressed in terms of a unit of time). 

Of course, the growth of the mouse population might have been the result of 30 female mice all 
having litters in a single month, together with some deaths scattered evenly over the rest of the 
year, just as the average speed of 20 miles per hour might have been achieved by driving 80 
miles an hour for a 3 -hour spurt and backing up slowly for the other 7 hours. In other words, the 
instantaneous rate of change in both cases may have been high and positive for a brief period and 
negative for the rest of the time. 

What we will calculate now is the instantaneous rate of increase (IRI) for a population—a 
quantity analogous to the readout on a speedometer, which tells how fast the car 


is going at a given instant. The IRI will be positive when the population is growing and negative 
when it is shrinking—a negative IRI being analogous to a speedometer reading when the car is 
backing up. 

It is reasonable to expect that the rate of change in the size of a population (let the size be N) in 
the course of time (t) will itself depend on the population size. (A mathematician would say the 
rate of change of /Vis a function ofN.) Whatever the average individual's contribution to 
population growth, it must be multiplied by the population size in order to determine the amount 
of change in the population as a whole. These simple relations, expressed in the notation of 
calculus, yield the most basic equation of population dynamics: - 

instantaneous average individual 

rate of = contribution to multiplied population 

increase population growth by size 


X N =rN 

Here dN/dt is the conventional notation for instantaneous rate of change of N with t. The average 
individual contribution (r) is the instantaneous rate of increase (per individual), usually called 
the intrinsic rate of increase by biologists. ^_Thus, at a given moment, if a population contains a 
million individuals (N) and is growing at that moment at the rate of 100,000 individuals per year 
(dN/dt), then r would be 0.10. In other words, the growth rate amounts to one-tenth of an 
individual per individual per year. In recent years the human population has been growing with 
an r of about 0.02; that is, roughly one-fiftieth of a person has been added per person per year to 
the total population. 

The factor r is more easily understood if it is split into its input and output components. If input 
is b (the instantaneous birth rate) and output is d (the instantaneous death rate), then r is seen to 
be simply b minus d. (We are ignoring migration.) If more individuals are being born at a given 
instant than are dying, the population is growing, and r is positive. If more are dying than are 
being born, the population is shrinking, and r is negative. 

Exponential Growth 

The equation dN/dt = rN relates the IRI to population size itself. It does not, however, show 
explicitly how the size of a population (growing according to this relationship) at one time is 
related to the size at another time. Consider, for instance, a population whose size at an initial 
time (t = 0) is known and is denoted N(0). How can one calculate the size t units of time later [N 
(t)]l ^_The answer can be derived from the equation for IRI by elementary calculus. For the case 
when r itself does not vary with time, one obtains: 

Size after 
f time 


initial multiplied 

f — 





N(t) = N(Q) 


e raised to the 

power of the 

IRI per capita 

multiplied by t 

v time units 



Here e is the base of the natural logarithm; it is approximately equal to 2.7183. When the 
quantity e is raised to a power that is a variable—for example, the product r?— the result is called 
the exponential function. It is sometimes also written, "exp(rt)." This function has unique 
mathematical properties that cause it to arise naturally in the mathematical representation of 
many physical and biological processes. Its presence in the growth equation just presented is the 
basis for calling this particular process exponential growth. 

Consider the application of the exponential growth equation to human population growth in 
India. The population of India in 1975 was about 620 million persons. How large would it be in 
2025-fifty years later—if it continued to grow at the 1975 rate (an r of roughly 0.026 per person 
per year)? Using the above formula: - 

N(0) = 620 million 

t= 50 yr; r = 0.026; rt = 1.30 

e rt = 2.7183 L30 = 3.67 

N(t) = 7V(50) = N(0)e rt = 3.67 X 620 million 
= 2275 million 

3 Some familiarity at least with the notation and fundamental approach of calculus is helpful for 
what follows. For an introduction or review, see E. Batschelet, Introduction to mathematics 
for life scientists, 2d ed. ( Springer-Verlag, New York, 1975), or a basic calculus text. 
This quantity is also sometimes called the innate capacity for increase. 

5 The alternate notations N o and N t are used in many biology texts. 

6 The symbol = means, "approximately equal to." 


BOX 4-2 Interest and Growth Rates 

If 7Y(0) dollars is placed in a bank at a 2-percent rate of interest, compounded semiannually, the 

7 1 

principal plus the interest at the end of t years will be N(t) = 7Y(0)(1 .01) . 

More generally, if the interest rate is expressed as a decimal r (where lOOr = interest rate as a 
percent) and interest is compounded x times a year, then 

N(t) = JV(0)I1 + -" 


If interest is compounded continuously, then x — > oo. Expression (1) maybe rewritten, 
substituting >> = r/x N(t) = 7V(0)(1 + yf' y = JV(0)[(1 + yf y = e. (2) 

Those who have had calculus will understand that as x — > oo, y — > 0, and in the limit 

lim (1 + y) Wv = e 

The base of natural logarithms (e) is approximately equal to 2.718. Therefore, when interest is 
compounded continuously, (2) becomes the familiar equation of exponential population growth 
N(t) = N(0)e rt . 

Given the growth rate, the doubling time may be computed by setting N(t)/N(0) = 2. Then 2 = e r . 

Taking the natural logarithm, (In) of each side (and remembering that In e = 1) gives In 2= rt or 

In 2 

= t 


= t 

For example, if the annual growth rate is 2 percent, then 1.02 = e r and r = 0. 0198, and 

0.6931 mm 
t = 35 yr 

0.0198 3 

The doubling time can be closely approximated by dividing seventy years by the annual growth 
rate in percent. (The approximation becomes less accurate as annual growth rate gets larger, 
however; above a rate of 10 percent it can be significantly in error.) 

Thus, in the year 2025 India's population would have grown to 2275 million (or 2.275 billion) 
persons— well over half the 1975 population of the entire Earth! 

A quantity growing exponentially increases in any given period of time by a fixed percentage of 
its size at the beginning of the period. A savings account at a bank where interest is compounded 
grows exponentially. The interest becomes part of the balance and, in turn, earns more interest. 
As time goes on, the balance gets bigger and the additions in the form of interest get bigger, in 
proportion. When growth is exponential, as it approximately is now in the human population, 
each addition becomes a contributor of new additions. The relationship between compound 
interest and population growth rates is made explicit in Box 4-2, as is a derivation ofe. 

If the percentage of exponential growth of a population in two successive periods is the same, the 
absolute growth in the second period is larger. Between 1960 and 1970, growing at about 2 
percent per year, world population increased by some 650 million persons; if the 2 percent 
annual rate of growth persisted through the 1970s, the population increase for that decade 
would be about 800 million. 

Exponential growth at a constant annual percentage increase can be characterized by the time it 
takes for any given quantity to double. The doubling time for a population growing at 2 percent 
per year is about 35 years. Consumption of energy— a useful index both of resource consumption 
and of a population's impact on the environment— is growing worldwide at roughly 5 percent per 
year, corresponding to a doubling time of only 14 years. (The general relation is: doubling time 
is approximately equal to 70 years divided by the annual percentage increase. See Box 4-2.) 

Use of the concept of doubling time emphasizes what for our purposes is the most important 
property of exponential growth— the rapidity with which such growth can exceed a given limit 
after seeming safely small for a long time. To understand this phenomenon, imagine a large 
aquarium with filter and aeration systems adequate 


to the needs of 1000 guppies. Suppose we start with 2 guppies in the tank and that the number 
grows exponentially with a doubling time of I month. It takes 8 doublings, or 8 months, for the 
guppy population to reach half the carrying capacity of the tank (2 — > 4 — > 8 — >• 16 —> 32 — > 64 
—> 128 —> 256 —> 512). For this entire period, the population seems safely small; no crisis seems 
imminent. Symptoms of impending disaster are unlikely to appear until the population is well 
over half the tanks capacity. The critical phase of the growth, when the population zooms from 
512 to more than 1000, occurs within the ninth month, when the last 100 guppies are added in 
less than 5 days. After 265 days of apparent prosperity, exponential growth carries the population 
from 90 percent of capacity to a disastrous excess in less than a week. 

If the subject is human beings instead offish in an aquarium, the limits are not so obvious, but 
the treacherous properties of exponential population growth are just as relevant. A long history of 
exponential growth does not imply a long future of exponential growth. 

Increase Without Restraint 

Restraints on the growth of a population include predators, disease, and shortages of resources 
such as food and water. When all such restraints are removed—when there are no predators or 
disease and there are superabundant resources~r reaches a theoretical maximum, symbolized as r 
m (m for "Malthusian"). Actually, r m for a given species is not a single number but has a range of 
values, depending on temperature, humidity, and other conditions. For a specified set of such 
conditions, then, the maximum possible exponential rate of growth is the corresponding 
maximum intrinsic rate of increase multiplied by the population size: 

dt i 



Suppose, for instance, that a grain beetle has established a population of 100 individuals in a bin 
of wheat in which the temperature is 29°C and the moisture content of the wheat is 14 percent. 
Assuming growth without significant restraint and an r m = 0.75 per individual per week, how 
many beetles would you expect to find after 3 weeks? The calculation is of the same form as that 
done for population growth in India, above—although the r for the Indian population was not the 
theoretical maximum (r m ) for the temperature and humidity of India! 

In the grain beetle case: N(0) = 100;r m = 0.75;and t = 3. And N(t) [ = N(3)] is to be calculated. 

After t weeks, 

N(t) - N(0)e Tmt 

After 3 weeks, JV(3) = 100e (075)(3) = 100e 225 = 949. 

Thus, one would expect to find 949 beetles after 3 weeks of unrestrained growth. After 10 weeks 
there would be 180,804 beetles; and in 20 weeks, 326,901,737 beetles. Essentially unrestrained 
growth obviously cannot continue for 20 weeks unless the wheat bin is large, since at that point 
more than 300 million beetles would be competing for the remaining grain. Indeed, unrestrained 
growth for a year would lead to: 

7V(52) = 100e (0 - 75)(52) =100e 39 

N(52) = 8,659,340,042,000,000,000 = 8.7 x 10 18 . 

If each beetle weighed 10 milligrams on the average, the entire mass of beetles would weigh 8.7 

1 1 78 

X 10 metric tons. After 82 weeks there would be 6.1 x 10 beetles, and their weight of 6.1 X 
10 metric tons would be equal to that of Earth! A general rule about long-continued 
exponential increase is that it leads to preposterous numbers with surprising speed. - 

Remember, in the more general case of exponential growth, where predators and disease are not 
eliminated (or in human populations, where some limitation of births is almost always practiced), 
r is substituted for r m . Hence, growth may be exponential at a rate below the theoretical 
maximum. For instance, the human population is now growing roughly exponentially with r = 
0.02 per person per year, whereas r m for human beings in most environments would probably be 
in the vicinity of 0.04 to 0.05. Exponential growth produces characteristic curves when numbers 
are plotted against time ( Figure 4-1 ) . These curves transform into a straight line if the 
population size is plotted on a logarithmic scale ( Figure 4-2 ), which is why it is often called 
logarithmic growth. 


The r m conditions in this example approximate those calculated for the grain beetle Calandra 
oryzae by L. C. Birch in Experimental background to the study of the distribution and 
abundance of insects, pt. 1, Ecology, vol. 34, pp. 698-71 1. 

40D r 


FIGURE 4-1 Exponential growth and arithmetic growth. A. Initial population of 100 million 

growing arithmetically at 1 million per year. B. Initial population of 100 million growing 

exponentially at I percent per year. C. Initial population of 50 million growing exponentially at 2 

percent per year. Compare A and B: The difference is negligible at first, then grows dramatically. 

This is the effect of compound interest. Compare B and C: A higher exponential growth rate 

soon compensates for a smaller initial population. 





FIGURE 4-2 Curves from Figure 4-1, replotted using a logarithmic scale on the ordinate. Note 
that only the previously straight arithmetic growth line, A, is now curved. 


It is important to keep in mind both the properties of exponential growth curves and the 
assumptions underlying them. Remember especially that the intrinsic rate of increase (r) is 
assumed to be constant and that the rate of change of population size (derivative of size with 
respect to time, dNIdt) is this constant, r, multiplied by the size at a particular time. Thus, when 
the beetle population was just 1000 individuals, the instantaneous growth rate was: 


= rN= 0.75(1000) = 750 individuals per week 

The human population in 1975 was some 4 billion persons and was growing approximately 
exponentially at about 2 percent per year. Hence, the midyear instantaneous growth rate was 
roughly 0.02 X 4 billion = 80 million per year. The relation between true instantaneous rates of 
change and the finite rates that can be computed from data generally available is explored in Box 

Growth with Restraint 

Suppose that, instead of being constant, an individual's reproductive contribution varies with the 
size of the population. Now the basic equation becomes: 

< rate of 




of V X population size 

population size 




where f(N) stands for a function of TV— that is, for r, which now varies with TV. 

The simplest mathematical model we can make of this situation is one in which r decreases 
linearly as TV increases. That is, in this model r is not a constant but rather is given by the 

r m\ 



Here K is the carrying capacity (the maximum number of individuals that can be supported in a 
given environment). According to this relation, at the beginning of the growth process (when N = 
0), r is equal to r m , the Malthusian rate of increase. Later, as N approaches the carrying capacity 
(K), r approaches zero. 

The differential equation for this simple model of growth is: 

This can be rewritten in the form: 

dN iK-W 


= N(a - bN) 

where a = r m and b = r m /K. By integrating this differential equation (expressing population size 
as a function of time), we obtain the following solution: 

1 + ce m 

where c = [K - N(0)]/N(0). 

This equation is that of the logistic curve ( Figure 4-3 ) -- the archetypal presentation of density 
dependence in ecology. Note in this figure that the growth rate of the population at each point in 
time depends on its density (size). Growth starts off slowly and then accelerates 

5 500 


£T 400 


t> 300 



FIGURE 4-3 The logistic growth of a population of the protozoan Paramecium aurelia in 

culture. Data show observed values that are quite close to the calculated logistic curve. (Adapted 

from Andrewartha and Birch, 1954.) 
-104- PAGENUMBER105 

BOX 4-3 Finite and Instantaneous Rates 

Finite birth and death rates are computed by dividing the number of births (or deaths) in a period 
by the average population size in that period (the sum of the initial and final sizes divided by 2) 
or by the size of the population at the midpoint of the period. These finite rates are estimates of 
the instantaneous rates, and the difference between the finite birth and death rates is an estimate 
of r, the instantaneous rate of increase per individual. 

Consider a population of rodents. Suppose there were 1 1,250 births and three-fourths as many 
(8,438) deaths in the population during a period in which the average population size was 
1 1,250. The finite birth rate would be 1 per individual per period, and the finite death rate would 
be 0.75 per individual per period. The instantaneous rate of increase per individual during that 
period in that case can be estimated to be 0.25 individuals per individual during the period—from 
which one could estimate that the final population would be 1 .25 times the starting size (the 
original number of individuals plus one-quarter of an additional individual for each of the 
original individuals). 

The factor by which the size of a population is multiplied during a given period is the finite rate 
of increase per individual, or, simply, the finite rate of increase (conventionally symbolized by 
the Greek letter lambda, X) for that period. Thus, if Nl is the population size at the end of the 
period and NO is the size at the beginning, 

N i = XN and 





What would X be in our example? To find out, we must establish the initial population size (No) 
and the final size (N i ). There were 1 1,250 births and 8,438 deaths, so 1 1,250 minus 8,438, or 
2,812, rodents were added to the population. We know the average population size during the 
period was 1 1 ,250, so the starting size (N o ) must have been 1 1 ,250 minus half the growth, or 
9,844, and the final size (N i ) must have been 1 1,250 plus half the growth, or 12,656. Thus: 

. Ni 12,656 , w 
X = — L = — — = 1.29 
N 9,844 

What accounts for the discrepancy between 1 .29 and 1 .25? The basic answer is that when a 
population is growing, the rate per individual gives a continuously varying total rate of increase 
because the number of individuals is steadily growing. Thus, an IRI per individual of 0.25 will 
multiply our hypothetical population by more than a factor of 1 .25 in a given finite period 
because the IRI per individual is applied not just to the original population of 9,844, but to a 
continuously growing population. 

The ratio X is known if the population sizes N o and N i are known, or it can be estimated by 
adding I to the difference between the finite birth and death rates for an interval, if those are 
known or can be estimated. In most theoretical work, such as mathematical models of population 
dynamics, one works not with X but with r, so it is necessary to know the relation between those 
two quantities. This relation can be obtained easily from the solution of the exponential growth 
equation. If the length of the time in question is one unit, we obtain: 

= \ = e 


In the example given here, X = 1 .29, so 
r = ln)i = In(1.29) = 0.255. 

Note that the estimate of r = 0.25, made by subtracting the finite birth rate from the finite death 
rate, differs from the exact value by only 0.005. When dealing in periods of time that are short in 
comparison with the doubling time of the population (that is, when finite rates of increase are 
very small), the instantaneous rate can be approximated by subtracting I from the finite rate. For 
example, rounded to two places: 

Estimate of r, True 

k-1 r 

1.01-1=0.01 0.01 

1.02-1 = 0.02 0.02 

1.03-1=0.03 0.03 

1.05-1=0.05 0.05 

1.20-1 = 0.20 0.18 

1.40-1 = 0.40 0.34 

2.00-1 = 1.00 0.69 

3.00-1=2.00 1.10 

10.00-1 = 9.00 2.30 

Therefore, when considering annual rates for humanity (where the finite rate is always below 
1.05), the difference between finite birth and death rates is a good estimator of the average r for 
the year. 

-105- PAGENUMBER106 

rapidly. The first part of the curve is sometimes called the log phase because, when plotted with 
N on a logarithmic scale ( Figure 4-2 ), it approximates the straight line of exponential growth. 
Then the curve rapidly flattens out, approaching K asymptotically (that is, the difference between 
N and K becomes infinitesimally small as t gets very large). 

There are many simplifying assumptions in the logistic equation. One is that all individuals are 
presumed to be alike ecologically (there is, for instance, assumed to be no change with age in the 
likelihood of giving birth or being eaten). There is assumed to be no time lag between a change 
in the environment and the reactions of the organisms—a very unrealistic assumption. Very low 

densities are presumed not to hinder mate-finding. And, most important, there are the 
assumptions that the carrying capacity (K) is constant and that r is directly proportional to K — N. 

Despite these simplifications, the logistic curve has proven to be remarkably close to observed 
patterns of population growth in laboratory cultures of organisms such as yeasts, protozoa, and 
fruit flies. Indeed, the recent history of human population growth might represent the log phase 
of a logistic pattern. It is not, however, known exactly what the carrying capacity is for the global 
human population, although it certainly is not a constant. As we discuss later, any determination 
of the carrying capacity for Homo sapiens must include the question of how long that carrying 
capacity is expected to persist. 

The question of changes in carrying capacity must be considered for any organism, although in 
laboratory experiments this may often be ignored. In nature, for instance, fruit flies may well 
show a logistic pattern of growth. A fertile female fly finds a suitable habitat—say, a pile of 
rotten fruit—and lays her eggs. The population then grows in a roughly logistic manner, but when 
the food is exhausted the population "crashes"— it is reduced to a very low level or goes extinct. 
This type of outbreakcrash population cycle is common in organisms that exploit temporary 
habitats. The species persists because dispersing individuals can find other suitable habitats. But 
Homo sapiens, as a whole, cannot follow this pattern, for there is only one known suitable 

The logistic model has also proven to be a valuable building block for the construction of 
multispecies models, which are a means of studying interactions among different organisms. For 
example, a simple model can be constructed for the growth of a pair of competing species, a and 
b, with two differential equations: 

dN. „ ( K a -N a -aN b 

^ = r N 

i, ma ai wr 


dN »- r N I K > - N » ~ W. 

dt - r "» N >\ K b 

The subscripts a and b indicate the intrinsic rates of increase, carrying capacities, and populations 
of species a and b. The Greek letter alpha (a) represents the degree to which the resources of 
species a are reduced by each individual of species b. The letter beta (a) is the degree to which 
the resources of b are reduced by each individual of a. In this model, a and (3 are constants and 
are called competition coefficients. Depending on the relationship of K a , K b , a, and (3, logistic 
competition can result in one species or the other wiping out its competitor or in the two species 
coexisting, depending on the initial proportions of the two populations. A considerable body of 

competition theory deals with such questions as the definition of ecological niche (the role an 
organism plays in an ecosystem), the degree of similarity possible between organisms in 
competition, and the circumstances under which they coexist or exclude one another. - 

Age Composition and Population Growth 

The assumption that all individuals are ecologically identical—equally likely, for instance, to give 
birth or die—is not even approximated in most animal populations and clearly is not the case in 
human populations. Understanding the effects of the age compositions of these populations (and 
changes in those compositions) on birth and death rates is critical to understanding their 
dynamics, especially when populations have overlapping generations. 

Consider the input side of the equation first. Given the requisite information, one can construct a 
schedule of age-specific birth rates for a population. In bisexual 

These matters are beyond the scope of this book but are dealt with in fine texts by Pianka and 
Roughgarden. ( E. Pianka, Evolutionary ecology; J. Roughgarden, Theory of population 

organisms, this schedule is the maternity function, mx (m for maternity, x for age). It is normally 
presented in terms of the number of female offspring per female. Such a function is plotted in 
Figure 4-4 for women in the United States. The shape of the curve is rather typical for human 
females— a rapid rise from the midteens to the late twenties, and then a sharp tapering-off around 
fortyfive. In populations in less developed countries, the entire curve extends farther to the left 
(reproduction starts earlier), and the peak usually is considerably higher because the average 
woman has more babies ( Figure 4-5 ). 

To make statements about population growth, however, we need to know more than what the 
number of female offspring per female will be in each age class. We also need to know how 
many females there are in each age class. For this, it is necessary to know the probability that an 
individual will survive to any given age (lx, in which 1 denotes living) which can be obtained 
from the age-specific death rates. Again, the pattern is similar for females of most human 
populations. As can be seen from the plot of age-specific death rates in Figure 4-6 , there is a 
burst of infant mortality, followed by low death rates through childhood and young adulthood, 
with a gradual increase starting in the late twenties. The basic U-shape of the curve holds for all 
human populations, but in less developed countries infant and child (ages 1-5) mortalities are 
considerably higher, and the high death rates of old age begin earlier. 

Conventionally, however, death-rate data are presented in terms of 1 x , defined either as the 
number of individuals surviving or the proportion of individuals surviving. Most animal 
populations show survivorship curves, (1 x ) of one of three types ( Figure 4-7 ). Human 
populations in developed countries come close to having a Type I curve, in which low early 
mortality is followed by a period of rapidly increasing death rates in middle age. The Type II 
curve, in which a constant proportion of the population dies in each age interval, is approximated 

by adults of some butterflies, adult fishes, some adult birds, and adults in primitive human 
groups (all of these organisms have high juvenile mortalities). Type III curves are probably the 
most common in natural animal populations: extremely heavy early mortality followed by a 
gradual die-off of the survivors. Most marine invertebrates, fishes, and insects show this general 





28 - 
12 - 
04 - 

i^ \^ [^ r^ \^ 1^ 

15-19 20-24 25-29 30-34 35-39 40-44 45-49 


Figure 4-4 The maternity function for American women, 1971. The ordinate indicates the 

number of offspring produced by an average woman in each period. Thus an average American 

woman reproducing at the 1971 rate would have 0.15 children between the ages of 20 and 24. 

(Data from U.S. Agency for International Development.) 






i 1 1 1 r 

19 20-24 25-29 30-34 35-39 40-44 45-49 


FIGURE 4-5 The maternity function for Peruvian women, 1969. If data were available, this 
curve would extend farther to the left than the one in Figure 4-4. (Data from U.S. Agency for 

International Development.) 


0- 1 


S 5- 10 10- 15 15 20 20-25 25- 30 30- 35 35-40 40-45 45-S0 v 6S ™ V S0-6S 

FIGURE 4-6 Age-specific death rates of American women, 1973. Only the prereproductive and 
reproductive years, critical to calculation of population reproductive rates, are shown in detail. 











" ■ ^Type » 

\Type II \ 

V Type III 


FIGURE 4-7 Three types of survivorship curve. Only the general shape of the curve is important 

here, not the scale. 

TABLE 4-1 

A Simple Life Table 


(Age class) 














(Average number 


of survivors) 

of offspring) 














If age-specific death- and birth-rate (/ x and m x ) data are available, it is possible to combine them 
in order to make some statements about population dynamics. Let us look at a single newborn 
female cohort (a group of females born at the same time) of a hypothetical animal. A version of a 
life table called an / x m x table can be constructed for the cohort by observing the pattern in which 
it reproduces and dies. In Table 4-1 , x is the designator of the age class (x - 1/2) to (x + 1/2) — 
that is, if x is 5 years, all individuals 4 1/2 to 5 1/2 years old are in Class 5. In the same table, / x , 

is the proportion of the cohort surviving to age x, and m x is the age-specific female birth rate. 
The key column is the right-hand one, the product of / x and m x . The sum of that column, 


, tells the average number of female offspring produced per female of the original cohort. This 
sum is designated R o and is known as the net reproductive rate (NRR). 

If R o= 1, as it does in this example, it means that the original female cohort has exactly replaced 
itself; for 1000 females in generation n, there are 1000 females in 


generation n + 1. If R o= 2, then in generation n + 1 there are 2000 females. If R o= 0.5, there are 
500 in generation n + 1. If generations do not overlap ~ that is, if all the parents die before any of 
their offspring nature (as in an insect with one generation per year) -then R0 indicates whether 
the population is growing, shrinking, or stationary (not changing in size). If R o is greater than 1, 
the population is growing; if R is less than 1, the population is shrinking; if R is exactly 1, the 
population is stationary. 

If generations overlap, however, no statement about population growth at a given moment can be 
made on the basis of a calculation of R o alone. It is necessary also to know the age composition 
of the population. The way R o is calculated in a population with overlapping generations is 
analogous to that of the preceding hypothetical example. Age-specific female birth- and death- 
rate schedules are determined for a given period of time; and those schedules are applied 
mathematically to a hypothetical cohort in order to calculate R o- The age composition of the 
actual population at the time of determination of the birth-rate and death-rate schedules is not 
taken into consideration. 

Suppose that R o in a human population is found to be 1 . Suppose further that almost half the 
women in the population are below reproductive age. Even if R o remained 1 and each generation 
were just reproducing itself, the population would continue to grow for some time because the 
proportion of females in their childbearing years would be increasing (raising the overall birth 
rate, b) more rapidly than the ranks of the elderly were increasing (raising the overall death rate, 
d). Remember that, ignoring migration, it is the relationship of these overall rates (not the age- 
specific rates) that determines whether the population is growing, shrinking, or stationary. 

The relationship between R o and r is discussed further in Box 4-4. Confusion of the net 
reproductive rate (R0) with the instantaneous rate of increase has, for example, led to many 
erroneous statements about population growth in the United States. When in the early 1970s the 
NRR was very close to 1 , many commentators declared that the United States had reached ZPG 
(zero population growth, a stationary population). Unfortunately, this was not so (see Chapter 5). 

What R o = 1 for a growing human population does mean is that if the age-specific vital (birth 
and death) rates that result in R o = 1 are maintained, the population will eventually reach ZPG 
(in about a lifetime). That is, if the NRR in the United States remained equal to 1 for some 
seventy- five years, and if there were no migration, the population would stop growing at the end 
of that time (ZPG, r = 0), and the size of the United States population at ZPG would be about 
280 million persons. 

The reason for the continued growth with NRR equal to 1 is that the age composition of the U.S. 
population in the early 1970s was not in a steady state. If the schedule of age-specific birth and 
death rates (and thus the NRR) subsequently remain constant, the age composition (or age 
distribution, as it is often called) will change gradually into what is known as a stable age 
composition or a stable age distribution. _When a stable age composition is reached, recruitment 
into each age class (births for the youngest, aging for the rest) is compensated by departures 
(mortality plus aging into the next older class) so the proportion of the population in each age 
class remains constant and each age class has exactly the same r as the entire population. This 
stability of age composition would be achieved in any population with constant vital rates; 
accordingly, a population with a stable age composition may be growing, shrinking, or 

Sometimes a population with such an age composition is referred to as a stable population. This 
should not be confused with a stationary population. A stationary population does not 
necessarily have a stable age composition (although, without a stable age composition, vital rates 
must keep changing in complex ways if the population is to remain stationary). Again, stable 
populations are not necessarily stationary, and stationary populations are not necessarily stable. 

It takes about the average life expectancy (symbolized e + 1)) for the age composition of a 
population to stabilize. The life expectancy can be calculated from a life table. Table 4-5 in Box 
4-5 shows this calculation for the female population of the United States in 1973. Since e +T) = 
75.3, obviously it would take about seventy-five years for the population of the United States to 
reach a 

9 A. J. Lotka, "The stability of the normal age distribution", Proceedings of the National 
Academy of Sciences, vol. 8 ( 1922), pp. 339-345. 
-109- PAGENUMBER110 

BOX 4-4 Generation Time and Calculation of r 

Given a / x m x table, and assuming a stable age distribution, r can be estimated easily. If T is the 
average length of a generation (that is, the average time between the birth of mothers and the 
birth of their female offspring), then the size of the female population after a generation of 
growth can be calculated: 

N , = N(0)e n , becomes N T = N(0)e T 


— £ = e rT = (by definition) the ratio of female 

^o births in two successive 


= R o = NRR. 

Therefore, R o = e , InR o = rT, and r = InR q/T. 

Thus, all that is required to estimate r is R oand an estimate ofT the generation time. 

How is the generation time estimated? It may be approximated from the information in the I x m 
table by the formula: 

T _ 2xl x m x _ ^xl x m x 
2l x m x R 

In this expression, xl x m x is the age of the female times the average reproductive contribution for 
that age class. The total of the product divided by the total reproduction of that generation (R q) 
gives the average age of females reproducing, weighted by the amount of reproduction of each 
age class. The value of T can now be used in the expression r = InR /T to calculate r. 

It is possible to get a more accurate estimate of the r associated with any I x m x schedule by 
substituting trial values ofr until a solution is found to Euler's formula: 

2°°_n/ rn e" rx = 1 

This involves calculating an e~' x value for each age class (x), multiplying by the Ixmx value, and 
summing the products. The procedure for getting an estimate of T and then r, and then using the 
latter as a starting point for trials for getting a more accurate r is shown in Table 4-2 . The data 
are for a hypothetical organism with overlapping generations, an R oof 1.85 and a generation 
time of about one and a half months. Only the first trial calculation is shown in the table 
(Columns 5 and 6). The calculated value for r (0.408) will, of course, only be realized when the 
age composition of the population is stable. 

Note that the more precise estimate ofr obtained with Euler's formula now permits a more 
accurate estimate of the generation time (T): 

r = 

0.408 = 

In /? 



T = 1.51 (0.10 months shorter than the 
original estimate, 1.61). 

The relationships ofR o, r, and T permit us to say something about the effects of age of 
reproduction on population growth. If reproduction occurs earlier, T becomes shorter; if 
reproduction is delayed, T naturally becomes longer. But how much effect this has on r depends 
on the magnitude of InR q. When R ois larger, a shift in generation time from, say, 20 years to 30 
years greatly reduces r (slows population growth). As R approaches 1, however, InR 
approaches 0, and the influence of an increase of T from 20 to 30 years is minimal. This can be 
seen in Table 4-3 , which shows the difference between r for 20and 30-year generation times and 
different values ofR o- 

From this result it can be seen that delaying the onset of reproduction (increasing T) may be a 
useful strategy for slowing the growth of a rapidly growing population; but the closer a 
population is to ZPG, the less effective this strategy will be. 

-110- PAGENUMBER111 

TABLE 4-2 


x l x 

m x 










6 (= 5 x 3) 


e l x m . 







1 0.80 






2 0.60 






3 0.30 






4 0.10 






5 0.00 








£/ Y m v = R o : 

= 1.85 

Ypcl x m x = 2.97 

1 2 3 (= 1 x 2) 4 (= x x 3) 5 6 (= 5 x 3) 

, , , -0.383a: -0.383a:, 

I x m x ' xtn x xl x m x e e I x m x 

T= 2.97/1.85 = 1.61 

r= 0.615/1.61 =0.383. 

When r = 0.383, 

When r = 0.383, Z? =0 l x m x e- r * = 1.034 

When r = 0.400, 

When r = 0.400, 2? =0 l x m x e- rx = 1.013 

When r = 0.410, 

When r = 0.410, 2f =0 l x m x e- rx = 0.995 

When r = 0.408, 

When r = 0.408, 2? =0 l x m x e- T ' = 1.001 

TABLE 4-3 

Effects of Generation Time (T) 

on r 


o ln^o 

ri(T = 

= 20) 


i(T = 

= 30) 

ri-r 2 




























BOX 4-5 Life Tables 

Age-specific death rates in a population ~ especially a human population ~ are often presented 
in the form of life tables. Such tables are of great interest to the actuaries employed by insurance 
companies and to government planners. Also, as described in the text, if the / x data in a life table 
are combined with the mx data of a fertility table, the net reproductive rate R0 is easy to 

The 1973 life tables for males and females in the United States are given in Table 4-4 and Table 
4-5. The source of the data is the United States National Center for Health Statistics. The 
explanation of the columns that follows is from the same source, slightly modified. Note, in 
particular, how the key variable, the life expectancy e + x, is calculated. 

Column 1, "Age Interval" (x-x + n). Column 1 gives the interval between the two exact ages 
indicated. For instance, "20-25" means the fiveyear interval between the twentieth birthday and 
the twenty-fifth. 

Column 2, "Proportion Dying" (nqx). This column shows the proportion of a cohort who are 
alive at the beginning of an indicated age interval who will die before reaching the end of that 
age interval. For example, for males in the age interval 20-25, the proportion dying is 0.01 1 1. Of 
every 1000 males alive and exactly 20 years old at the beginning of the period, 11.1 will die 
before reaching their twenty-fifth birthdays. The nqx values represent probabilities that persons 
who are alive at the beginning of a specific age interval will die before reaching the beginning of 
the next age interval. They are the age-specific death rates. The "Proportion Dying" column 
forms the basis of the life table; the life table is so constructed that all other columns are derived 
from it. 

Column 3, "Number Surviving" (lx). This column shows the number of persons, starting with a 
cohort of 100,000 live births, who survive to the exact age marking the beginning of each age 
interval. The lx values are computed from the nqx values, which are successively applied to the 
remainder of the original 100,000 persons still alive at the beginning of each age interval. Thus, 
of 100,000 male babies born alive, 98,022 complete the first year of life and enter the second; 
97,674 begin the sixth year; 96,405 reach age 20; and 13,662 live to age 85. In life tables the lx 
schedule is often given as the probability of reaching an age interval (the proportion of 
individuals reaching that interval), as in Table 4-1. 

Column 4, "Number Dying" (ndx) shows the number dying in each successive age interval out of 
100,000 live births. Of 100,000 males born alive, 1978 die in the first year of life, 348 in the 
succeeding four years, 1068 in the five-year period between exact ages 20 and 25, and 13,662 die 
after reaching age 85. Each figure in Column 4 is the difference between two successive figures 
in Column 3. 

Columns 5 and 6, "Stationary Population" (nLx and Tx). Suppose that a group of 100,000 
individuals is born every year and that the proportion that dies in each such group in each age 
interval throughout the lives of the members is exactly that shown in Column 2. If there were no 
migration and if the births were evenly distributed over the calendar year, the survivors of those 
births would make up a population that is both stationary and stable. It is stationary because in 
such a population the total number of persons is constant, and stable because the number of 
persons living in any given age group is constant. When an individual leaves the group, either by 
death or by growing older and entering the next higher age group, that person's place would 
immediately be taken by someone entering from the next lower age group. Thus, a census taken 
at any time in a stationary and stable community would always show the same total population 
and the same numerical distribution of that population among the various age groups. In such a 
stationary population, supported by 100,000 annual births, Column 3 shows the number of 

persons who, each year, reach the birthday that marks the beginning of the age interval indicated 
in Column 1, and Column 4 shows the number of persons who die each year in the indicated age 
interval. The sum of Column 4 must of course be 100,000, or the population would not be 

Column 5 shows the number of persons in the stationary population in the indicated age interval. 
For example, the figure given for males in the age interval 20-25 is 479,389. This means that in a 
stationary population of males supported by 100,000 annual births and with the proportion dying 
in each age group always in accordance with Column 2, a census taken on any date would show 
479,389 persons between exact ages 20 and 25. 

Column 6 shows the total number of persons in the stationary population (Column 5) in the 
indicated age interval and all subsequent age intervals. For example, in the stationary population 
of males referred to in the discussion of Column 5 preceding, Column 6 shows that there would 
be at any given moment a total of 4,808,61 1 per- 


TABLE 4-4 

Life Table of Males in the United States, 1973 




of years 



at the 


of the 


Of 100,000 born alive Stationary population interval 









at the 


of the 



who die 







1 2 






JC~ JC ' Yl fi is x 


n f* x 

n*-' x 

T x 

e+ x 

0-1 0.0198 






1-5 0.0035 






5-10 0.0024 












at the 



of years 

of the 

of life 




at the 

who die 




of the 

interval that 





000 born alive 

Stationary population 


















































































































sons who had passed their twentieth birthdays. The population at all ages and older (in other 
words, the total population of the stationary community) would be 6,756,651. 

Column 7, "Average Remaining Lifetime" (e + x). The average remaining lifetime (also called 
expectation of life) at any given age is the average number of years remaining to be lived by 
those surviving to that age, calculated on the basis of a given set of age-specific rates of dying. In 
order to arrive at this value, it is first necessary to observe that the figures in Column 5 of the life 
table can also be interpreted in terms of a single lifetable cohort without introducing the concept 
of the stationary population. From this point of view, each figure in Column 5 represents the total 
time (in years) lived between two indicated birthdays by all those reaching the earlier birthday 
among the survivors of a cohort of 100,000 live births. Thus, the figure 479,389 for males in the 
age interval 20-25 is the total number of years lived between the twentieth and twentyfifth 
birthdays by the 96,405 (Column 3) males (of the 100,000 born alive) who reached the twentieth 

birthday. The corresponding figure in Column 6 (4,808,61 1) is the total number of years lived 
after attaining age 20 by the 96,405 reaching that age. This number of years divided by the 
number of persons surviving (4,808,61 1 divided by 96,405) gives 49.9 years as the average 
remaining lifetime of males at age 20. 



BOX 4-5 (Continued) 

TABLE 4-5 

Life Table of Females in the United States, 1973 






at the 




of the 

of years 


of life 



who die 

at the 




interval that 

of the age 



Of 100,000 born alive 

Stationary population 


















x-x + n 

« (/ X 


n f* X 

n*-' x 

T x 

e+° x 










































































at the 


of the 



who die 



of years 

of life 


at the 


I that 

Of 100,i 

000 born alive 

Stationary population 


of the age 

































































stable age composition once the vital rates became constant. And, with NRR = 1, population 
growth would not cease until the age composition stabilized. 

Furthermore, only when there is a stable age composition is a population growing truly 
exponentially, that is, according to the equation 

N(t) = N(0)e rt . 

In many human populations, growth is close enough to exponential that if you know r — that is, b 
— d — you can calculate a doubling time (Box 4-2) without significant error. Where the age 
composition is far from stable, however, the calculated r and doubling time would be very 
misleading. For instance, in the United States in the early 1970s, there was a sudden change in 
net reproductive rate, and the age composition was very different from the stable composition 
that ordinarily would be associated with the new NRR. Thus, while the difference between birth 
rate and death rate in 1974 (about 0.6 percent) indicated a doubling time of approximately 120 
years, in fact, if the NRR remained constant, population growth would have stopped in about 
seventy-five years with less than a 50 percent increase in population. 


If you read Box 4-3, you saw that the finite rate of increase (X) can in theory vary from zero 
(when the population goes extinct) to plus infinity (the population size is increasing infinitely 
rapidly). Between zero and one the population size is shrinking; at one it is stationary (neither 
growing or shrinking); and above one it is growing. By contrast, the instantaneous rate of 
increase r is between zero and plus infinity if the population is increasing, zero and minus 
infinity if it is decreasing, and zero if it is stationary (if it has reached ZPG). 

Population Structure 

Very frequently ecologists wish to know what factors are influencing population size. They may 
need the information to design a system for exploiting a valued resource such as a stock of food 
fishes. Or they may wish to manipulate the population sizes of other economically important 
organisms — for instance, to suppress a dangerous crop pest or a vector of disease, or to increase 
the population size of a pollinator or a predator on pests. Whenever the dynamics of an organism 
is to be investigated for any reason, some of the most important data are those giving information 
on population structure. In the most general sense, population structure is simply all the factors 
that influence the way in which matings occur in a population. It includes such elements as age 
composition, mating preferences, and the patterns of distribution and movement. The latter 
patterns are considered in this section. 

Suppose the populations of two species, the white rhinoceros of Africa and the Sumatran 
rhinoceros of Southeast Asia, were reduced to seventy- five individuals each. Which would have 
the best chance of survival? Everything else being equal, you would have to bet on the white 
rhino because of its population structure. It lives in herds, so mates would have little problem in 
finding one another (often a difficulty when a population size drops below a critical point). By 
contrast, the Sumatran rhino lives alone in territories, and mating apparently occurs by chance 
encounter during an annual period of wandering over huge areas. _^_Thus, reduction in 
population size is bound to have more serious consequences for this rhino than for the white 
rhino. Indeed, this species is severely threatened today — its numbers have been reduced to 
perhaps between 100 and 170 individuals, scattered over much of Southeast Asia, where 
expansion of human populations has led to a rapid destruction of suitable habitat. Furthermore, 
like other Asian rhinoceroses, the Sumatran rhino is still hunted because of the use of its horn, 
hide, most organs, and even urine, by Chinese pharmacists. (Many Chinese, who make up a 
substantial portion of the Southeast Asian population, hold a traditional belief in rhinoceros horn 
as a potent aphrodisiac.) — 

It is not uncommon to find many different population structures in the same group of organisms. 
In California one kind of checkerspot butterfly, Euphydryas editha, tends to divide into many 
rather small populations that rarely exchange individuals. A nearly indistinguishable species 
occurring in the same area, Euphydryas chalcedona, moves around more, and its populations 

1 7 

occupy larger areas. Both of these species have small, restricted populations, however, in 

contrast to a nearly ubiquitous subalpine butterfly of the Rocky Mountains, Erebia epipsodea. 
Populations of E. epipsodea cover such enormous areas that individuals separated by more than 
10 kilometers may be members of the same interbreeding unit, whereas individuals ofE. editha 
separated by only 0.5 kilometers may belong to different units. — 

A totally different pattern has been found in the longlived tropical butterfly Heliconius ethilla. 
The temperatezone butterflies mentioned above apparently wander more or less at random over 
areas where nectar sources for the adults and food plants for the larvae are widely distributed. 
The Heliconius, by contrast, have relatively ritualized daily movements. Much of their time is 
spent moving among widely scattered plants from which the adults collect pollen. The pollen 
provides amino acids which allow the butterflies to live for several months as adults. Individual 
Heliconius set up "trap lines" and move in patterns determined by whatever plants are blooming 
at a given time. Population units seem to consist of clus- 

10 G. E. Hutchinson and S. D. Ripley, Gene dispersal and the ethology of the Rhinocerotidae, 

Evolution, vol. 8 ( 1954), pp. 178-179. 
n J. Fisher, N. Simon, and J. Vincent, Wildlife in danger, Viking, New York, 1969. 
12 Paul R. Ehrlich, R. White, M. Singer, S. McKechnie, and L. Gilbert, "Checkerspot butterflies: 

A historical perspective", Science vol. 188 ( 1975), pp. 221-228 (April 18). 

1 ~\ 

P. F. Brussard, and Paul R. Ehrlich, Contrasting population biology of two species of 
butterfly, Nature, vol. 227 ( 1970) pp. 1 19-129. 

100 200m 


Territoriality in the song sparrow. The map shows dimensions and positions of territories on a 

tree-dotted floodplain at Columbus, Ohio (April 6, 1932). Broken lines enclose territories of 

forty- four males. (Adapted from Nice, 1937.) 






X x 

x X 



x , 




X X 



x X 


X > 






M w 






















" X 



































Kinds of distribution, Each X indicates the position of one hypothetical individual. Note that the 

number of empty squares increases as one goes from overdispersed to random to clumped 

distribution (each example includes thirty individuals). 

ters of more or less circular trap lines, and movements of more than 400 meters are apparently 


rare. — 

The patterned movements of Heliconius butterflies are also characteristic of some tropical bees 
with widely scattered pollen resources. But such movements are much more common among 
vertebrates, where an individual concentrates its activities within a single area, the home range. 
Frequently the home range, or a portion of it, is defended against other members of the same 
species or, more rarely, members of other species. The defended portion of the home range is 
called a territory. Territoriality may be a year-round phenomenon, as it is with many small 
pomacentrid fishes of coral reefs, individuals of which defend a small area of the reef against all 
comers (including human beings 25 or more times their length!) the year round. Other organisms 
may defend their territories for only a brief, specific period, usually the breeding season. For 
instance, many male birds maintain territories during the breeding season (their songs are 
announcements of intent to defend and readiness to breed), but they flock together peacefully 
when not breeding. Homo sapiens — popular literature to the contrary — does not show individual 
territoriality in the same sense that these other animals do. — 

The reasons that territoriality evolved seem varied and as yet are not entirely understood. But at 
least one result of territoriality is clear ~ it tends to space individuals more or less evenly over an 
area ( Figure 4-8 ) . Since most territories can only be compressed so far, this also tends to limit 
the maximum size of a population, often keeping it well below the limits of food resources. 
Much may be inferred about the behavior of particular animal populations merely by plotting 
their distributions (Figure 4-9). If individuals are clumped ( Figure 4-9 A ), they are probably 
social; if they are overdispersed ( Figure 4-9C ), territoriality is likely; and if they are randomly 
distributed ( Figure 4-9B ), they probably have a minimum of interaction. There are statistical 
tests that can distinguish these three states, tests that are applied to counts of individuals in grid 
squares. If there are some squares with many individuals and many with none, it indicates social 

14 Paul R. Ehrlich and L. E. Gilbert, Population structure and dynamics of the tropical butterfly 

Heliconius ethilla, Biotropica, vol. 5 ( 1973), pp. 69-87. 
15 "For a discussion of territoriality in human beings and its relationship to territoriality in 

general, see E. O. Wilson, Sociobiology. 
-116- PAGENUMBER117 

if most squares have one or a few individuals, that indicates territoriality. But the scale of the 
grid must be selected with care. On the scale of a single woodland, a population of tree-nesting 
birds might seem overdispersed; but on the scale of a state they might appear to be clumped, 
because woodlands are clumped. 

Dispersion is the term for the three patterns of individual distribution just discussed; dispersal is 
the movement of individuals into and out of population units. Knowledge of dispersion can have 
practical application in interpreting the dynamics of populations that are important to humanity. 
Information on dispersal is important because, as we discuss later, invasions of communities by 
alien organisms may have a dramatic effect upon those communities, often to the detriment of 
humanity. The ability of organisms or of their propagules (eggs, seeds, and such) to move is their 
vagility. The chances for successful dispersal of an organism from one area to another depends 
on the vagility of the organism, the presence of barriers (for example, water for a terrestrial 
organism, lowlands for a mountain organism, dry land for a fish), and the availability of suitable 
habitat in the new area. 

Mankind, of course, has played a major role in the dispersal of organisms, especially in the past 
century. Many plants have seeds that stick in the coats of passing mammals and are thus 
dispersed. Such seeds also tend to be dispersed readily on the clothes people wear. Other plants 
have moved around the world as seeds mixed in with the ballast of ships or with fodder carried 
for domestic animals on ships and trains. And, of course, many plants have been purposely 
transplanted, often carrying with them unwanted insects or other animals. People have also 
purposely dispersed animals, frequently with unhappy results (see "Extinction, Endangered 
Species, and Invasions," below). 

Data on the Dynamics of Natural Populations 

Unfortunately, relatively few sets of data on natural populations are useful in testing the theory 
of population dynamics. There are several reasons for this. First, censusing animals is usually 
difficult. Rarely is straightforward counting possible, so various procedures for estimating 
numbers must be employed. One technique used frequently is mark-release-recapture analysis. 
Individuals are marked in some way (with butterflies, coded numbers are put on the wings with 
felt-tipped pens), released, and allowed to mix back in with the population. Later a second 
sample group is taken. Suppose 100 individuals were captured and marked the first time, and, of 
a later sample of 100 captured, 10 had been marked previously. Then, assuming the equivalence 
of the ratios 

population size (N) 
sample marked (5, ) 

second sample taken (S 2 ) 

number of marked recaptures (R) 

we can estimate N as 

N = 


Or, in our example: 

N = '°° X '°° = 1000 


This simple estimate is based on the assumption that if 100 marked individuals (marks) made up 
10 percent of a population, the population must have 1000 individuals. Of course, in real life 
things are not so simple — statistical errors enter with the sampling, the marks may not mix 
randomly with the remainder of the population or they may be more or less readily captured than 
unmarked individuals, and there may be recruitment or loss to the population between the first 
and second samplings. Various procedures have been developed to help compensate for these 
problems, and despite the difficulties, mark-release-recapture analysis is widely used. 

In addition to estimates of population size, this technique also provides information on 
population structure — for instance, the statements about butterfly population structures made 
earlier were based on experiments in which many thousands of individuals were marked and 
released. Data obtained from these experiments can also be used to estimate survivorship and 
longevity. For example, while temperate-zone butterflies live as adults for an average of perhaps 
two weeks, the tropical Helicoius ethilla are much longer lived — averaging about two months, 
with some surviving five months or more. 






5000 - 
4000 - 

3000 - 
2000 - 








-i — I — I — r — r — r 

i — i — i — i — r 

/ A 

i960 1962 1964 1966 1968 1970 1972 1974 


5000 - 
4000 - 

3000 - 
2000 - 

"S 1000 - 
£ 800 

£ 600 








<10 - 

J I I I I 1 1 L 

J I I I L 

1960 1962 1964 1966 1966 1970 1972 1974 

B FIGURE 4-10 Changes in the size of the Jasper Ridge colony of checkerspot butterflies, 

Euphydryas editha. A. Three populations (areas C, G, and H) are plotted separately. There are 

discrete generations, one per year. Parents and offspring never fly together. Note two extinctions 

and one reestablishment of G population. B. The total size of the Jasper Ridge colony 
(populations C, G, and H summed). The extinctions are not detectable, so important information 

on the dynamics of the three units is lost. 

The temperate-zone species described have only one generation per year, so their generations do 
not overlap. The Heliconius do have overlapping generations; grandparents fly with their adult 

The importance of understanding population structure when interpreting data on population 
dynamics can be seen by examining changes in the size of the Euphydryas editha populations 
diagramed in Figure 4-10 . When studies of the dynamics of E. editha were begun in 1960, it was 
thought that there was only a single population in the Jasper Ridge biological experimental area 
on Stanford University's campus. A capture-recapture analysis that year revealed that what had 
been thought to be a single population were three quite isolated populations. These populations 
occupied different parts of an island of grassland surrounded by chaparral (see the discussion 
under "Biomes," below) and were not separated by any obvious barriers. _^_What would have 
happened had this structure been overlooked and die entire three-population colony been treated 

as a dynamic unit? For one thing, the extinction and reestablishment of Population G would not 
have been detected, and for another the constancy of the overall population size would have been 
exaggerated (note that the line in Figure 4-10 showing changes in the total size of the colony is 
much smoother than that of any of the individual populations). 

Why is this so important? Most of the effort in population ecology goes into attempting to 
understand why populations change in size as they do. Ecologists wish to understand how 
various environmental factors influence population size so they can predict future sizes and - 
where it is economically important — manipulate factors to influence future population sizes. 
They wish to reduce populations of pests and increase populations of beneficial organisms. One 
of the main ways of finding out what affects population size is to correlate observed changes in 
size with observed changes in an environment. But, as with the E. editha, if two or more 
populations are lumped together, an erroneous picture of changes in population size may be 
drawn. This is especially critical because it is thought that the influence of environmental factors 
on populations size is in part a function ofpopula- 

16 Paul R. Ehrlich, The population biology of the butterfly, Euphydryas editha: pt. 2, The 
structure of the Jasper Ridge colony, Evolution, vol. 14 ( 1965), pp. 327-336. 
-118- PAGENUMBER119 

1 7 

tion size; that is, the populations respond to the environment in a density-dependent manner. — 
For instance, the larger an animal population is, the less likely a given individual is to have an 
abundant food supply or a satisfactory place to shelter from the weather. Thus the availability for 
each individual of both food and shelter — as well as other resources — may be density- 

The more that environmental factors have densitydependent effects, the less variation you would 
expect in the size of a population. As a population increased, the life of the average individual 
would become increasingly precarious, reproduction would diminish, and the population size 
would begin to fall. When it decreased far enough, resources per individual would become 
abundant again, life would be more secure, and the population would tend to increase again. The 
more sensitive the operation of the factors to density, the less variation may occur in population 
size. Thus, erroneously lumping populations C, G, and H of Jasper Ridge E. editha would 
produce data much more indicative of a densitydependent "regulation" of population size than 
would be warranted by the actual behavior of the individual populations. 

There is no more enduring controversy in population ecology than that over whether the sizes of 

1 R 

populations are generally regulated in a density-dependent manner. Ecologists who work with 

insects tend to believe that the feedback effect from density on population size is minimal. Those 
who study territorial organisms like birds are often impressed by the way availability of space for 
territories regulates the sizes at least of breeding populations in a density-dependent fashion. An 
additional complication is that populations differ genetically and may change in their genetic 
compositions from generation to generation. (This concept is discussed further under "Natural 
Selection and Evolution.") Some workers claim that there is genetic feedback, because the 
genetic makeup of a population affects its density. Changes in population density affect the 

environment, which in turn (through natural selection) changes the genetic makeup of the 

Detailed studies ofE. editha populations indicate that, within the same species, density may have 
very little influence on population size in some cases, whereas in other populations it may be 
very important. Furthermore, there is evidence that the importance of density may change from 
generation to generation. 19 It seems likely that few great generalities about modes of population 
regulation are possible. It is obvious that at some point the growth of any population will be 
halted by factors operating in a density-dependent manner, because no resource is infinite. It is 
also obvious that the carrying capacity will always vary through time. But, below that point, the 
degree to which density at generation n will influence density at generation n + \ appears to vary 
greatly among organisms, among populations of the same organism, and even, in some cases, 
between generations of the same population. Beyond that, population ecology may have to 

concentrate on developing procedures for investigating dynamics. The reasons that populations 

of both Australian magpies ( Figure 4-11 ) and Heliconius ethilla ( Figure 4-12 ) are relatively 
constant in size may be quite different, but the approach to discovering what the critical factors 
are and making predictions about population sizes in the future can be very similar in both cases. 

The definition of population units, for instance, is important in both cases. (This, we must repeat, 
is basic. For example, failure to determine whether the Peruvian anchoveta fishery is exploiting 
one population or two has made rational planning of that exploitation difficult.) A search for 
limiting factors can then be made. (It looks as if space for territories, operating in a 
densitydependent fashion, is the critical factor for the magpies. The situation of the Heliconius 
butterflies is more complex, but adult pollen resources appear to limit the population, also in a 
density-dependent fashion.) 

1 7 

Population density is technically the number of individuals per unit area, but, since most 
populations have limited space, an increase in population size automatically means an 
increase in density. Thus densitydependent and population-size-dependent are usually 
synonymous, and the former is the accepted expression. 

1 R 

An excellent, brief historical summary of the controversy can be found in Krebs, Ecology, p. 

Ehrlich et al., Checkerspot butterflies. 

The landmark publication in this area is H. G. Andrewartha and L. C. Birch, The distribution 
and abundance of animals. This book is a must for all serious students of ecology. Among 
other things, the authors' classification of environmental factors into four components (1) 
weather, (2) food, (3) other animals and pathogens, and (4) a place to live was a conceptual 
breakthrough. A broader classification that can be used for plant populations as well appears 
in Ehrlich et al., The process of evolution, p. 84. 





5 eo 

























1957 1956 1959 I960 1961 

FIGURE 4-11 Changes in the size of a population of magpies, Gymnorhina tibicen, near 

Canberra, Australia. Individual birds may live for many years. The broken line indicates a 

significant gap in sampling. (Data from Carrick, Proceedings of the Thirteenth International 

Ornithological Congress, 1963, p. 750.) 



l I I I I I — I t i i t i — i — rr 


40 - 
30 - 

20 - 

<10 ■ 

t — i — r~[ — i — i — r 

Dry Mtisn 


Dry season 

J L_l I I i ■ ■ 

J I L 

J — I — I l_L 



120 160 200 240 260 220 360 400 440 

Dale Dec 69 May "70' July 70 'Aug TO Feb 71 Oct 70 Dec 70 Match 72 

FIGURE 4-12 Changes in die size of a population of Heliconius butterflies, H. ethilla, in the 
mountains of northern Trinidad. Adults are long-lived (up to six months), and generations 
overlap; grandparents may fly alongside their grandchildren. 
Multi-Species Systems 

Very often population ecologists are concerned with the simultaneous behavior of two 
interacting populations. A great deal of attention has been paid, for example, to predator-prey 
systems. In part this is a response to such fascinating phenomena as the partially linked cyclic 
changes in the abundance of prey and specialist predators—Canadian snowshoe rabbits and 
lynxes, for example. In part it results from the need to understand such systems when attempting 
to introduce predators as biological controls of pests. Of most direct interest in human ecology 
are the interacting dynamics of human populations with parasites such as the plague bacterium 

9 1 

and the influenza virus. — 

Sometimes the dynamics of two or more populations are intertwined in a competitive situation. 
Competition occurs whenever two or more organisms utilize a common resource that is in short 
supply. Both predation and competition have been studied extensively in the laboratory, since 
both appear to be very important in determining the structures of natural communities. For 
example, competition between two species of flour beetles, Tribolium castaneum and Tribolium 
confusum, was stud- 

9 1 

See K. E. F. Watt, Ecology and resource management, chapter 5. 
-120- PAGENUMBER121 

ied in a series of experiments. The basic technique was to start mixed colonies in containers of 
flour and then see which species won the competition by eliminating its competitor under various 
conditions of initial density, volume of flour, temperature, humidity, the presence or absence of a 
parasite, and so forth. (A win invariably occurred — the two species will not coexist.) 

In experiments in which temperature and humidity were varied, it was found that under some 
conditions T. castaneum always won; under other conditions T. confusum always won; and in 
intermediate conditions the outcome was "indeterminant" — sometimes T. castaneum won and 


sometimes T. confusum. ~ The indeterminant results proved to be caused by genetic differences 
in competitive ability within the species of beetles. Beetles can be selected for ability to compete; 
strains exist of T. castaneum, for instance, that invariably win in the temperature-humidity range 

previously thought to be indeterminant. The outcome under intermediate conditions was 

actually determined by the genetic characteristics of the individuals used to start each culture. 

One of the shortcomings of ecology in its earlier days was a tendency to ignore the genetic 
properties of populations under study (it was geneticists, not ecologists, who first pointed out 
indeterminacy as a probable cause in the flour beetle experiments). In order to understand certain 
key aspects of ecology, it is important to have at least a passing acquaintance with the genetic 
properties of populations and how they change through time — that is, an acquaintance with 
natural selection and evolution. 

The Dynamics of Exploited Populations 

Often the ecologist interested in population dynamics is asked to recommend a pattern of 
exploitation of some economically valuable animal population such as a fishery. Although an 
extensive and complex literature exists on this topic, we can only mention a few points about it 

here. ^_First of all, just what form the exploitation will take must be decided. In many, if not 
most cases, the problem from a biological point of view seems to be to design an exploitative 
program so that a maximum sustainable yield (MSY) is obtained. A maximum sustainable yield 
is the greatest amount of a population being exploited (fish, whale, deer, or whatever) that can be 
harvested without reducing future yields. MSY can be defined in terms of total numbers 
removed, total biomass removed, numbers of a particular age class removed, or biomass of some 
age class. Obtaining each of these would ordinarily require quite different harvesting strategies. 
In addition, the exploiter must choose either to maximize the productivity of the population or to 
minimize the loss to other causes of mortality. In the first case, harvesting might be concentrated 
on reproducers, minimizing competition for those remaining and inducing high productivity. In 
the second, harvesting might be concentrated on juvenile individuals because they are most 
subject to natural mortality. 

Consider a simple model for the harvesting of a population following a logistic growth curve 
(Figure 4-13). A glance at the curve shows that dN/dt is maximal around the middle, where the 
increasing upward push ofr m N is balanced by the increasing downward push of (K — N)/K. (K. 
= carrying capacity.) Under this model, the MSY (numbers) could be harvested merely by 
removing enough individuals to keep the population size stationary at that point. A sustainable 
yield (SY, numbers) could be obtained by keeping the population larger or smaller, but in both 
cases the SY would be smaller than the MSY. Regardless of the strategy of exploitation, the 
MSY is always obtained by keeping populations below their maximum potential sizes, because 
production is higher when the population is smaller. 

The model presented for MSY makes a number of biological assumptions — such as that the 
stock being 

T. Park, Experimental studies of interspecies competition: pt. 2, Temperature, humidity and 
competition in two species of Tribolium, Physiological Zoology, vol. 27 ( 1954), pp. 177-238. 

9 3 

I. M. Lerner and F. K. Ho, Genotype and competitive ability of Tribolium species, American 
Naturalist, vol. 95 ( 1961); pp. 329-343; J. Park , P. H. Leslie, and D. E. Mertz, Genetic 
strains and competition in populations of Tribolium. Physiological Zoology, vol. 37 ( 1964), 
pp. 97-162. For a fine general treatment of both competition and predation, see the Krebs text, 
Ecology, chapters 12 and 13. 


For example, see R. J. N. Beverton and S. J. Holt, On the dynamics of exploited fish 
populations, H.M. Stationery Office, London, 1957; or E. D. LeCren and M. W. Holdgate, 
eds., The exploitation of natural animal populations, Blackwell, Oxford, 1962. A student 
interested in the dynamics of exploited populations should also refer to Krebs, Ecology, and 
Watt, Ecology. 



















40i i: i 









FIGURE 4-13 Sustainable yields in a hypothetical population growing according to the logistic 
model. The IRI (dN/dt) is indicated by tangents to the curve at three points SY1, MSY, and SY 2 - 

Note that the maximum sustainable yield (MSY) is obtained at the point where dN/dt is 

maximum, that is, when the population consists of about 5000 individuals. Sustainable yields at 

lower (SY i ) and higher (SY 2 ) population sizes would be smaller. 

exploited is relatively self-contained, that K is more or less constant, and that patterns of density- 
dependence of reproduction do not cause large fluctuations in the amount of stock. It does not 
consider the social organization of the exploited population or the possible effects of exploitation 
on the age structure of the population. When such factors are ignored, a pattern of harvesting can 
lead to a population crash from which no recovery occurs. 

More serious, however, is the total absence from the model of economic considerations such as 
the costs of harvesting, the elasticity of demand for the product, and whether the population 
exploited is common property (as in an oceanic fishery) or controlled by an owner (a commercial 

fish pond). When economic factors are included, harvesting is often pushed beyond the 
biological MSY, since by doing so an economic variable such as receipts can be maximized. — 


Natural selection is the creative process in evolution. It is essentially the differential 
reproduction of genetic types. In most animal and plant populations and in all human 
populations, individuals differ because each individual has a different hereditary endowment. For 
instance, people differ in such traits as eye color, height, and blood type, all of which are at least 
partially hereditary. If people with one hereditary trait (that is, one kind of genetic information) 
tend to have more children than those with another, then natural selection is occurring with 
respect to that trait. Natural selection can cause one kind of genetic information — for example, 
that producing people with a certain type of hemoglobin — to become more and more common in 
the pool of genetic information (the gene pool) of a population. This might occur as a response to 
an environmental change in which mosquitoes carrying malaria become more common, since 
individuals with one kind of hemoglobin are more 

See, for example, J. A. Crutchfield and G. Pontecorvo, The Pacific salmon fisheries: A study 
of irrational conservation, pp. 28-36. 
-122- PAGENUMBER123 

resistant to malaria than those with another. Changes in the gene pool of a population constitute 
the basic process of evolution. 

It is evident that in the evolutionary history of humanity, for instance, there was a selective trend 
toward increased brain size. This is a convenient shorthand for a more complete description that 
might go like this: 

In early human populations there was variation in brain size. This variation was largely caused 
by differences among individuals in their genetic endowment. Individuals with slightly larger 
brains were presumably better able to utilize the cultural information of the society. This 
permitted them readier access to mates, a better chance of surviving, or perhaps a better chance 
of successfully rearing their offspring; and they reproduced more than did individuals with 
smaller brains. The result was a gradual increase in the genetic information producing larger 
brains in the gene pools of human populations. In turn, this increased the capacity for storing 
cultural information and thus produced a selective advantage for further increase in brain size. 
This reciprocal evolutionary trend continued until other factors, such as the difficulty of getting 
the enlarged braincase of a baby through the female's pelvis (which was not commensurately 
enlarged) at birth, removed the selective premium on further increase in brain size. 

Understanding natural selection will help to illuminate a number of points in this book. Chapter 
1 1, for example, discusses the development of resistance to pesticides in insect populations, a 
process that occurs through natural selection. Individual insects often vary in their natural 
resistance to a pesticide, and this variation has a genetic basis. Insects that are naturally more 
resistant have a better chance of surviving, and thus of reproducing, than their less fortunate 

fellows. In this way an entire population may become more and more resistant when sprayed 
repeatedly with a pesticide, as in each generation the most resistant individuals do most of the 

A key concept to bear in mind is that natural selection is the differential reproduction of genetic 
types {genotypes, in the shorthand of geneticists). Natural selection often involves differential 
survival, but differentials in reproduction may occur even when the life expectancies of all 
genotypes remain identical. All may live the same length of time, but some may be relatively 
sterile while others are highly fertile. Another key point to remember is that natural selection 
cannot operate unless there is genetic variability in a population. If there is no variation, all 
individuals are genetically identical and there can be no differential reproduction of genotypes. 
Such a population would be unlikely to survive for long, since it would lack the ability to make 
evolutionary adjustments to changed conditions. 

The environmental factors that lead to natural selection are called selective agents. Changing 
weather can be a selective agent, leading to an accumulation of genotypes that thrive under new 
climatic conditions. DDT can be a selective agent, leading to insect populations made up of 
genotypes resistant to DDT. (In that case, humanity was the ultimate selective agent, but the 
resultant evolution was undesired.) 

Artificial selection occurs when human beings purposely arrange for the differential reproduction 
of genotypes. Artificial selection is practiced by all plant and animal breeders as they control the 
differential reproduction of genotypes. The idea is to develop strains in which the characteristics 
most valuable to society are maximized — weight in swine, milk production in cows, egg 
production in chickens, shape and durability in tomatoes, height above ground for ears of corn 
(for ease in harvesting), beauty in flowers, and so on. These strains have undergone evolutionary 
processes that adapt them for a man-made environment. 

Without human assistance, these strains would disappear through extinction or reversion under 
the countervailing pressure of natural selection. That pressure must always be reckoned with by 
the plant or animal breeder. Frequently, attempts to enhance a single characteristic too much 
upset development and diminish fertility; thus, natural selection opposes further progress under 
the pressure of artificial selection. 

Like natural selection, artificial selection can occur only if the requisite genetic variation is 
present. Biologists and agronomists regard genetic variation in animals and plants as an 
invaluable resource that must be preserved so that new strains can be selected to meet new needs. 
As we discuss later, however, some of the richest 


sources of that variation in important crop plants are being lost at an alarming rate (see Chapter 

Evolutionary forces other than natural selection also can change the gene pools of populations. 
Mutation can change one form of a gene into another and thus change the constitution of the 

gene pool. Immigrants bringing genes into the pool of a population or emigrants taking them out 
may have genetic constitutions different from the population as a whole, and thus migration can 
change the pool. Both mutation and migration may increase the genetic variability of a 

Furthermore, chance occurrences, which are inevitably involved in the passage of genetic 
information from one generation to the next, can also lead to changes in the gene pool. Without 
going into detail, one can think of the adults of generation n + 1 as possessing a sample of the 
genetic information present in those of generation n. Just as a sample of 100 flips of an honest 
coin usually will not produce exactly 50 heads because of sampling error, so the gene pool of 
generation n + 1 will not be exactly the same as that of generation n, because of sampling error. 
Because sampling error causes random changes in the frequency of genes in a pool, it is referred 
to as genetic drift. 

The magnitude of genetic drift in a population is itself a function of the population's size. The 
smaller the population, the more important drift is (just as the fewer the flips of the honest coin, 
the larger the deviation from 50-50 is likely to be). In any sampling, some of the genes present in 
generation n may not be present in generation n + 1 — that is, some of the genetic variability may 
be lost. Such decay of variability is especially severe in small populations — making it difficult, 
say, to maintain the variability of crops by storing small samples of seed. — 

If a population is greatly reduced by natural catastrophe or hunting, it is likely to suffer a decay 
of genetic variability. If, for instance, a given gene is represented on the average in 1 of 10,000 
individuals, it is highly likely that gene will be lost if the population is reduced to 100 
individuals. This loss of genetic variability may greatly reduce the chances that the population 
can evolve appropriately in response to environmental changes and thus enhances the probability 
of extinction. 

Even if a population increases again in size after undergoing such a genetic bottleneck, the gene 
pool may remain impoverished for a considerable period. For instance, the northern elephant 
seal, which lives on the coast of California and Baja California, went through such a bottleneck 
in the nineteenth century. Its population ~ now more than 30,000 individuals — may have been 
reduced to as few as 20 individuals in the 1890s. Investigations of the genes controlling a variety 
of enzymes indicate that the genetic variability of the population is much less today than that of 
its subantarctic relative, the southern elephant seal, and that of other vertebrates, and it may 
therefore be highly vulnerable to environmental change. — 

Although mutation, migration, and genetic drift are all evolutionary forces, it is important to 
remember that natural selection is the creative force in evolution. It is selection that shapes 
populations and species in response to changing environments. And, as will become apparent in 
the next section of this text, selective interactions are major forces in the evolution of 
communities. — 


Predator-prey systems illustrate a situation in which one can expect each population to affect the 
evolution of the other. When two populations are ecologically intimate, each exerts selective 
pressure on the other. (See Box 4-6 for descriptions of some other forms of ecological intimacy.) 
Hence, differential reproduction induced by a predator should bring about improvement in the 
escape mechanisms of the prey, and, reciprocally, selection in the prey should sharpen a 
predator's attack. 

To put it more precisely, in each generation those genetic variants among the predators that are 

26 Selection can also reduce variability. Any program of maintaining living samples of 
organisms to preserve variability inevitably selects for individuals that culture well. This, in 
turn, tends to reduce variability further. 

M. L. Bonnell and R. K. Selander, "Elephant seals: Genetic variation and near extinction", 
Science, vol. 184 ( 1974), pp. 908-909 (May 24). 

This discussion of evolution is necessarily greatly simplified and abbreviated. Those 
interested in more detail may wish to consult Ehrlich et al., The process of evolution, 
especially chapters 6 and 7, which deal with population genetics. 
-124- PAGENUMBER125 

BOX 4-6 Ecological Intimacies 

A rather complex terminology has developed around various kinds of relationships between 
species. Some of the terms most commonly used in the literature are defined below. 
PREDATOR. Usually this is applied to an animal that kills and devours other animals {the prey). 
The term is increasingly being used also in place of herbivore (plant-eater), especially when the 
result is the demise of an individual plant (as in seed-predator). HERBIVORE. An animal that 
eats living plants, whether or not it kills them. PARASITE. An organism that feeds on another 
organism {host) but normally does not kill it (or kills it only very slowly). Parasites are usually 
much smaller than their hosts, and predators are often larger than their prey, but the dividing line 
between predator or herbivore and parasite is sometimes blurred. For instance, parasites usually 
live out their lives in or on a single host individual, but some insects are confined to a single 
plant and are generally called herbivores. PARASITOID. Usually this term is applied to insects 
that lay eggs on, in, or near other insects or spiders, which are their hosts. The young parasitoids 
live inside the host, inevitably killing it about the time the parasitoids form their pupae (a resting 
stage of many insects between larval and adult stages). Like parasites, parasitoids are smaller 
than their hosts, often more than one lives on a single host, and they do not kill their host 
quickly. Like predators, they always kill their hosts. Some parasitoids are important agents in 
biological pest-control programs (see Chapter 11). 

MODEL. A distasteful or dangerous organism that another species mimics in order to escape 
from predation. Models usually have aposematic (warning) coloration that makes them obvious 
and "advertises" their obnoxious character. In Batesian mimicry the mimic is tasty or harmless. 
In Mullerian mimicry models mimic each other, presumably so that predators see the same 
advertisement associated with obnoxiousness more frequently. Predators then need to sample 
fewer prey in order to learn the meaning of the advertisement than they would if each model had 

a different aposematic pattern. MUTUALISM. Any mutually beneficial association, such as that 
between two Mullerian mimics or that between a rhinoceros and a tick-bird (the bird gets food by 
cleaning ticks off the rhinoceros, and the rhinoceros gets rid of its parasites). 
COMMENSALISM. An association of two species in which one benefits and the other is not 
harmed. Tiny mites living in the facial hair follicles of most people with healthy skin are good 
examples of commensals. Most persons are utterly unaware of the presence of these harmless 
residents. SYMBIOSIS. A blanket term for parasitism, commensalism, and mutualism; in older 
literature it is sometimes used as a synonym for mutualism. COMPETITION. The utilization of a 
resource in short supply by two or more individuals or populations. Competition may be 
interspecific (among individuals of differing species) or intraspecific (among individuals of a 
single species). 

successful in attacking the prey are likely to be most successful in reproducing, and thus the 
efficiency of the predator can be expected to increase generation by generation. This does not 
mean, however, that soon the predator will be so effective that the prey population will be wiped 
out, for the prey evolves too. In each generation, the prey individuals that escape predation are 
the ones to reproduce and contribute their genes to posterity. 

This kind of point-counterpoint selective process was originally described in detail for butterflies 
whose caterpillars (herbivores) eat plants (prey). The kinds of reciprocal evolutionary changes 
found in that system and in similar systems (such as predator-prey, host-parasite, model-mimic, 

and competitor-competitor systems) were christened coevolution. It was pointed out that in 

such systems the populations were often involved in a coevolutionary race in which extinction of 
one or the other element was a possibility. The predator, for instance, could become so efficient 
as to wipe out the prey — or the prey so elusive as to starve the predator. 

Paul R. Ehrlich and P. H. Raven, Butterflies and plants: A study of coevolution, Evolution, 
vol. 18 (1964), pp. 586-608. 

FIGURE 4-14 Stone plants (Lithops julii var. reticulata) growing in the desert at Karasburg, 
Southwest Africa. Various succulent plants of the family Aizoaceae have evolved extraordinarily 
effective camouflage that makes them closely resemble small stones. (Photo courtesy of Chester 


Understanding the coevolutionary aspects of ecological systems is extremely important to 
anyone interested in human welfare. The importance can be seen clearly in the relationship 
between plants and the organisms that attack them. Plants cannot escape their predators by 
running away, so they have evolved a variety of other defenses. Some, like the stone plants of 
African deserts ( Figure 4-14 ), have evolved camouflage patterns that make them very difficult 
to see. More familiar are mechanical defenses against herbivores, especially the sharp spines of 
such plants as cacti and acacia trees. But the most nearly ubiquitous and seemingly most 
effective defenses of plants are biochemical. 

Plants have evolved a vast array of compounds, such as alkaloids, terpenes, essential oils, 
tannins, and flavonoids, raphides (needlelike calcium oxalate crystals), and other crystals, all of 
whose roles appear to be to poison or otherwise discourage the animals, fungi, and bacteria that 
prey on the plants. These compounds are used by humanity in many ways. They are the active 
ingredients in spices (many essential oils), medicines (quinine, belladonna, digitalis, morphine), 
stimulants (caffeine), and other drugs (nicotine, marijuana, opium, peyote). Plant compounds are 
even used for their primeval purpose — as pesticides (nicotine, pyrethrin). 

Insects and other predators on plants have been engaged in a long coevolutionary race with the 
plants, evolving strategies to avoid being poisoned. Undoubtedly, some have become extinct, but 
many others have persisted, some running the race so well that they are able not only to eat the 

poisonous plants, but in some cases even adopt the poisons as part of their own defenses against 
predators. For example, the monarch butterfly (Danaus plexippus) contains vertebrate heart 

poisons which it obtains from its milkweed food plants. The poisons cause the blue jay 

predators of the monarch to vomit. Considering their evolutionary experience with poisons, it is 
not surprising that many wild populations of insects have become resistant to DDT and other 
pesticides as a result of the indiscriminate use of those poisons. 

Plants have been shown to suffer heavy losses to insect predators, and yet there clearly are 
physiological limits to how much poison the plants can sequester without poisoning themselves 
or without allocating to poisonproduction too much of the energy they need for growth, 
maintenance, and reproduction. At least some plants have shown an interesting evolutionary 
response to this problem. In the Rocky Mountains of Colorado, populations of lupines and other 
leguminous plants suffer heavy attacks from the larvae (caterpillars) of small blue (lycaenid) 
butterflies. These butterflies, with a name longer than their wingspread (Glaucopsyche 
lygdamus), lay their eggs on the flower buds ( Figure 4-15 ), and their larvae devour the flowers. 
Of some inflorescences (groups of flowers on a common stalk), 85 percent or more of the 
flowers are destroyed, with a corresponding loss in seedset. 31 Therefore, considerable selective 
advantage should accrue to any plant genotypes that are resistant to the insect's attack. 
(Remember, reproductive success is the result that counts in evolution.) By counting the eggs 

T. Reichstein, J. V. Euw, J. A. Parsons, and M. Rothschild, "Heart poisons in the monarch 
{Danaus plexippus)" , Science vol. 161 ( 1968), pp. 861-866; and L. P. Brower and S. C. 
Glazier, "Localization of heart poisons in the monarch butterfly", Science, vol. 188 ( 1975), 
pp. 19-25. 

D. E. Breedlove and Paul R. Ehrlich, Coevolution: Patterns of legume predation by a lycaenid 
butterfly, Oecologia, vol. 10 ( 1972), pp. 99-104. 
-126- PAGENUMBER127 

laid on inflorescences in several lupine populations, it was determined that the level of 
Glaucopsyche attack was much higher in some than in others. A careful analysis of the alkaloid 
content of plants from the various populations gave a clue to one major reason for this 
variability. Populations that for other reasons were unavailable to the butterflies contained small 
amounts of a single alkaloid. Those exposed to attack accumulated much higher amounts of 
alkaloids in their inflorescences. Of this exposed group, the populations that suffered heavily 
from predation contained a mixture of nine alkaloids that varied very little from plant to plant. 
The populations that were exposed but remained relatively free of attack by Glaucopsyche, in 
constrast, showed great variation from plant to plant in the mixtures of three or four alkaloids 


they contained and in the total quantities of alkaloid in their inflorescences. — 

From these and related data, it was hypothesized that the variability in their alkaloid content was 
itself a defense mechanism against small specialized herbivores like Glaucopsyche. By 
presenting the butterfly population with a varied defense, the plants were able to retard the 
development of resistance to that defense. Butterflies that as larvae developed on one alkaloid 
type were certain to lay many of their eggs on other types. Most of the offspring of a larval 
generation selected by one poison regime would themselves face a different regime. 

How did the plants invent such an ingenious strategy? It obviously is a result of the 
coevolutionary process. The more common an alkaloid type is, the more insects feed on it for 
several consecutive generations and thus become relatively resistant to that particular type. That 
type therefore suffers the heaviest seed loss and thus becomes less common in the lupine 
population. Thus, & frequency-dependent selection situation presumably is set up, in which 
variability is maintained because the heaviest predation is directed at the most common type. 

The direct importance to agriculture of understanding this sort of coevolutionary system is 
obvious. It is entirely possible, for instance, that by deliberately introducing and maintaining 
variable biochemical defenses 

FIGURE 4-15 A lycaenid butterfly, Glaucopsyche lygdamus, laying an egg on a lupine bud, 
Gunnison County, Colorado. (The body, behind the wing, curves down at left to deposit the egg.) 
The larvae of this tiny butterfly (wingspread about 25 mm) are major predators on lupine plants. 

(Photo by P. R. Ehrlich .) 

in crops, the need to use dangerous synthetic pesticides might be dramatically reduced. The 
potential for erecting such chemical defenses is especially high in crops like cotton, which 


people do not eat (cotton is the major crop in terms of insecticide use). But with sufficient 

cleverness such defenses might be adapted for some food crops. 

Similarly, it is important to keep in mind the defensive functions of many, if not all, "secondary" 
compounds of plants — that is, those with no obvious role in normal life processes of the plant. 
There is, for instance, an active research program underway to breed a strain of cotton whose 
seeds do not contain the poisonous compound gossypol. This would be a financial bonanza for 
cotton growers, since without gossypol, edible cottonseed protein could be prepared cheaply 
enough that the crop could probably earn a profit as a food crop, with the lint 


" P. M. Dolinger, Paul R. Ehrlich, W. L. Fitch, and D. E. Breedlove, Alkaloid and predation 
patterns in Colorado lupine populations, Oecologia, vol. 13 ( 1973), pp. 191-204. 

33 About 50 percent of the pesticides used in the United States are applied to cotton and tobacco 
( D. Pimentel, Extent of pesticide use, food supplies, and pollution). 

(fiber) serving as a bonus. But gossypol probably evolved as cotton's main chemical defense 
against insects and other pests. Without it, the already difficult pest problems of cotton might 
become insuperable. 

Plants do not necessarily fight the coevolutionary battle with insects and other pests by 
manufacturing poisons. Nitrogen needed by seedlings is stored in grain seeds in the form of 
amino acids, the building blocks of proteins. These amino acids are essential nutrients for all 
animals, which must use them to make their own proteins. But the balance of amino acids in 
grains is not ideal for utilization by human beings and many other animals — in grains, for 
example, the essential amino acid lysine tends to be in especially short supply. This makes the 
seeds less nutritious both for people and for other predators, and may well be the reason low- 
lysine seeds evolved. Work is progressing among agronomists to select new high-lysine strains 
of grain as a partial answer to world protein shortages. Unfortunately, these efforts may only be 
short-circuiting a major defense of the grains against herbivores, thus creating pest problems that 
might overbalance the advantages of the new strains. ^_The attempts to find new strains of both 
cotton and grain may be worthwhile, but in the process careful attention should be paid to 
possible coevolutionary constraints on their success. 


The plants, animals, and microorganisms that make up a biological community in an area are 
interconnected by an intricate web of relationships, a web that also includes the physical 
environment in which the organisms exist. You will recall that the interdependent biological and 
physical components make up what biologists call an ecosystem. The ecosystem concept 
emphasizes the functional relationships among organisms and between organisms and their 
physical environments. These functional relationships are exemplified by the food chains 
through which energy flows in an ecosystem, as well as by the pathways along which the 
chemical elements essential to life move through that ecosystem. These pathways are generally 
circular ~ the elements pass through the system in cycles. The cycling of some elements is so 

slow, however, that in the time-span of interest to society, movement appears to be one-way. An 
understanding of the flow of energy and the cycling of materials in ecosystems is essential for 
perceiving what may be the most subtle and dangerous threat to human existence. This threat is 
the potential destruction, by humanity's own activities, of those ecological systems upon which 
the very existence of the human species depends. 

In Chapter 3 we discussed the physical aspects of nutrient cycling in detail, but we dealt with the 
biological components of the ecosystems as black boxes. Now the time has come to pry the lids 
off those boxes and examine the living parts of those systems. A logical place to start is with the 
feeding sequences ecologists call food chains. 

Food Chains 

All flesh is grass. This simple statement summarizes a basic principle of biology that is essential 
to an understanding of both ecological systems and the world food problem. The basic source of 
food for all animal populations is green plants — "grass." Human beings and all other animals 
with which we share this planet obtain the energy and nutrients for growth, development, and 
sustenance by eating plants directly, by eating other animals that have eaten plants, or by eating 
animals that have eaten animals that have eaten plants, and so forth. 

One may think of the plants and animals in an area, together with their physical surroundings, as 
comprising a system through which energy passes and within which materials move in cycles. 
Energy enters the system in the form of radiation from the sun. Through the process of 
photosynthesis, green plants are able to capture some of the incoming solar energy and use it to 
bond small molecules into the large (organic) molecules that are characteristic of living 
organisms. That process is often called fixing the solar energy. Basically, the energy from light is 
used in a complex process in which water and carbon dioxide (CO 2 ) are raw materials, and 
glucose (a 

34 Such problems have already been reported (see L. R. Brown, By bread alone, Praeger, New 
York, 1974, p. 165). 

sugar) and oxygen are the ultimate end products. The chemical bonds of the products contain 
more energy than those of the raw materials; that added energy is derived from the sun. — 

The plants themselves and the animals that eat plants break down the large organic glucose 
molecules and put to use the energy that once bound those molecules together. The process by 
which they mobilize the energy is called respiration (or, more precisely, cellular respiration) 
(see Figure 4-16 ). In outline, respiration is the opposite of photosynthesis — glucose is combined 
with oxygen (oxidized) to produce water and carbon dioxide, and the energy thus released is 
trapped in chemical bonds of special molecules that transport it to other points of use. 36 The 
animal or plant expends some of that energy in its daily activities and uses some of it to build 
large molecules of animal or plant substance (for growth or repair of tissue). Animals that eat 

other animals, once again break down the large molecules and put the energy obtained 
that originally arrived in the form of solar energy — to their own uses. 


According to the first law of thermodynamics (see Box 2-3), energy can be neither created nor 
destroyed, although it may be changed from one form to another (as in the change from light 
energy to the energy of chemical bonds in photosynthesis). The second law of thermodynamics 
says, in essence, that in any transfer of energy there is a loss of available energy; that is, a certain 
amount of the energy is degraded from an available, concentrated form to an unavailable, 
dispersed form. The practical expression of this law as it applies to food production is that no 
transfer of energy in a biological system is 100 percent efficient; some energy always becomes 
unavailable at each transfer. In the photosynthetic system, usually I percent or less of the sunlight 
falling on green plants is actually converted to the kind of chemical bond energy that is available 
to animals eating the plants. Usually something less than 10 percent of the energy in 


CO* + H*0 — [CHaOV- + 0? 

[CH*0> + Oa -* CO* + H*0 


FIGURE 4-16 

Photosynthesis and respiration are the basic metabolic processes of most living plants, which 

obtain their energy from sunlight. In photosynthesis, energy from light is used to remove carbon 
dioxide and water from the environment; for each molecule of CO 2 and H 2 O removed, part of a 

molecule of carbohydrate (CH 2 O) is produced and one molecule of oxygen (O 2 ) is returned to 

the environment. In respiration by a plant or an animal, combustion of carbohydrates and oxygen 

yields energy, CO 2 , and H 2 O. (Adapted from Gates, 1971.) 

the plants is converted into chemical bonds in animals that eat the plants. Roughly 10 percent of 
that energy may in turn be incorporated into the chemical bonds of other animals that eat the 
animals that ate the plants. 

Thus, one may picture the flow of energy through this system as a step-by-step progression along 
what is known as a food chain. A food chain starts with the green plants, which are known as the 
producers or autotrophs (selffeeders). They are the first trophic (feeding) level. At all other 
trophic levels are the heterotrophs (other- feeders). At the second trophic level are the herbivores 
(plant-eating animals), the primary consumers . At the third trophic level are the secondary 
consumers, the carnivores (meateaters), which eat herbivores. Tertiary consumers are the 
carnivores that eat other carnivores, and so forth. Human beings play many roles in the food 
chains, but 

For a fine elementary discussion of the photosynthetic process, see P. H. Raven and H. Curtis, 
The biology of plants, 2d ed., Worth, New York, 1976. For an excellent, more detailed 
treatment see P. M. Ray, The living plant, 2d ed., Holt, Rinehart and Winston, New York, 
36 For details, see Paul R. Ehrlich, R. W. Holm, and M. E. Soule, Introductory biology, 
McGraw-Hill, New York, 1973, or any other modern biology text. 


f^- 1 

. #* ; ^ t tr 


(H3£CT| sj ^^T 


I „■■. ^ 

FIGURE 4-17 
A food chain including a man. A mosquito feeding on the man would be a quaternary consumer. 
-130- PAGENUMBER131 

their most common role is as a herbivore, since grains and other plant materials make up a very 
great proportion of the diets of most people. People can also be secondary consumers, as when 
they eat beefsteak (or the meat of any other herbivorous animal). When they consume fishes, 
they often occupy positions even farther along the food chain, because many fishes are tertiary or 

even quaternary consumers themselves. A food chain including human beings is pictured in 
Figure 4-17 . 

At each transfer of energy in a food chain, perhaps 90 percent of the chemical energy stored in 
the organisms of the lower level becomes unavailable to those of the higher level (in certain 
situations the percentage lost may be much greater or less than this). Since the total amount of 
energy entering the food chain is fixed by the photosynthetic activity of the plants, obviously 
more energy is available to organisms occupying lower positions in the food chain than is 
available to those in higher positions. For instance, as an oversimplification, it might take 
roughly 10,000 pounds of grain to produce 1000 pounds of cattle, which in turn could be used to 
produce 100 pounds of human beings. By moving people one step down the food chain, ten 
times as much energy would be made directly available to them ~ that is, the 10,000 pounds of 
grain used to produce 1000 pounds of cattle could be used instead to produce 1000 pounds of 
human beings. 

It follows from this application of the second law of thermodynamics that in most biological 
systems the biomass (living weight) of producers is greater than that of primary consumers; the 
biomass of primary consumers in turn is greater than that of secondary consumers; and so forth. 
The weight of organisms possible at any trophic level is dependent upon the energy supplied by 
the organisms at the next lower trophic level; and some energy becomes unavailable at each 
transfer ( Figure 4-18 ). 


Let us review for a moment two key concepts in the energetics of communities that were first 
discussed in connection with the carbon cycle (Chapter 3). Gross primary production (GPP) is a 
measure of the total 

O A • 





PRODUCERS- ..' iVV'-ita--" V-->" ■*.' 





c- ■> > •' *■ * * ' 

r dT fc ^ ^ y f< ■» 

FIGURE 4-18 A. An intact natural ecosystem exemplified by a mature oak and hickory forest 

that supports several levels of consumers in the grazing food chain, with 10-20 percent of the 

energy in each trophic level being passed along to the next level. The symbols represent different 

herbivore and carnivore species. Complexity of structure regulates population sizes, maintaining 

the same pattern of energy distribution in the system from year to year. B. An agricultural 

ecosystem is a special case, yielding a larger than normal harvest of net production for 
herbivores, including human beings and the animals that provide meat for them. Stability is 
maintained through inputs of energy in cultivation, pesticides, and fertilizer. (Adapted from 

Woodwell, 1970.) 

energy fixed by a community through the process of photosynthesis. Net primary production 
(NPP), on the other hand, is the GPP minus the energy used by the plants themselves for 


respiration. The GPP for the entire Earth is on the order of 1018 kilocalories. The global 


E. P. Odum, Fundamental of ecology. In a concession to tradition, in this section of text we 
are using kilocalories (kcal) instead of kilojoules (KJ) as the units of energy. The coversion is 
4.18 KJ = 1 kcal. 

TABLE 4-6 

Primary Production and Biomass Estimates for the Biosphere 


2 3 4 

Total net 







Net energy 

mass, 10 

Area, Mean net production, 




10 km primary C/yr 


10 kcal/yr 


tons C 


= productivity, (columns 2 


(columns 4 




10 m g C/m /yr x 3) 

kcal/g C 


kg C/m 


Tropical rain 

17.0 900 15.3 






7.5 675 5.1 






2 3 4 5 6 7 8 


Total net plant 

primary Net energy mass, 10 9 

Area, Mean net production, fixed, Mean metric 

10 km primary C/yr Combustion 10 kcal/yr plant tons C 

Ecosystem = productivity, (columns 2 value, (columns 4 biomass, (columns 

x 3) kcal/g C x 5) kg C/m 2 2*7) 

10.6 31 16 80 


10 n m' 

g C/m /yr 














Boreal forest 




Woodland and 

l 8.0 











Tundra and 






Desert scrub 




Rock, ice, and 








Swamp and 




Lake and 








Open ocean 












Algal bed and 




10.2 39 13.5 95 





































10.0 5 0.3 2.4 

























Total net 


Net energy 





mass, 10 


Mean net 





10 6 km 2 




10 kcal/yr 


tons C 




(columns 2 


(columns 4 




r/ii2 2 
10 m 

g C/m /yr 


kcal/g C 


kg C/m 










Total marine 








Full total 








All values in columns 3 to 8 are expressed as carbon on the assumption that carbon content 
approximates dry matter x 0.45. 

Grams of carbon per meter per year. 

Source: Whittaker and Likens in Carbon and the biosphere, G. M. Woodwell and E. V. Pecan, 

NPP — the energy available to all food chains — has recently been estimated to be about 7.2 x 

1 n 

10 kilocalories, or roughly 720 million billion ki 
NPP varies greatly from ecosystem to ecosystem. 

10 kilocalories, or roughly 720 million billion kilocalories. _ As Table 4-6 shows, however, the 

Some additional terminology is commonly used in discussions of community energetics. Net 
community production (NCP) is calculated by subtracting from the NPP the part of that 
productivity used by the heterotrophs in the community. In crop ecosystems, a farmer normally 
tries to maximize all three kinds of productivity as well as the ratios of NPP/GPP and NCP/GPP. 


In other words, a crop is usually selected that will fix as much solar energy as possible and use 
as little energy as possible in respiration. Heterotrophs other than human beings are excluded 
from the system insofar as is possible. Table 4-7 shows a comparison of a crop ecosystem (an 
alfalfa field) and a natural ecosystem (a tropical rain forest). 

In grazing agricultural ecosystems, on the other hand, human beings often try to maximize the 
secondary productivity of some heterotrophs such as cattle. In such a case, the NPP/GPP ratio 
may be high, but the NCP/GPP ratio will be low. Suppose two grazing food 


R. H. Whittaker and G. E. Likens, Carbon in the biota. See also H. Lieth , Primary 
productivity in ecosystems: Comparative analysis of global patterns , for an estimate about 6 
percent higher. For an estimate some 50 percent higher, see L. E. Rodin, N. I. Bazilevich, and 
N. N. Rozov , Productivity of the world's main ecosystems. Both of these papers and the 
volumes they are in will be very useful for those interested in the problems of evaluating 


The measurements, of course, are done before people, as the ultimate heterotrophs in the food 
chain, take their share. 

chains were observed: Grass 1 — » cattle — > people; and Grass 2 —> cattle — > people. One might 
well be interested in the ecological efficiency with which the two chains operated. This could be 
calculated by determining how many kilocalories of grass were eaten by the cattle and dividing 
that by how many kilocalories of beef were consumed by the people. Presuming that the people 
in both groups ate the same portions of the cattle (and that everything else was equal), this 
calculation would tell which of the two grasses was best to plant. Efficiency of energy transfer 
between trophic levels is called a Lindeman 's efficiency and has the general form: 

energy intake at nth trophic level 
energy intake at (n — l)th trophic level 

Ecological efficiencies may also be calculated within trophic levels in the form: 

net secondary productivity at nth level 
energy intake at nth level 

A rule-of-thumb figure of 10 percent has already been given as the Lindeman's efficiency. 
Measurements vary, however, from community to community and between trophic levels. If 
anything, they are likelier to be slightly more than the 10 percent, although they are rarely more 
than 25 percent. Yet efficiencies as great as 70 percent have been reported for some marine food 
chains. — 

Efficiencies within trophic levels vary a great deal, but tissue-growth efficiences tend to decrease 
as one goes up the food chain. [Tissue-growth efficiency is the ratio of net productivity at level n 
to assimilation (energy fixed in photosynthesis or food absorbed from the alimentary canal) at 
the same level.] For entire communities these efficiencies seem to average around 60 percent for 
plants, about 40 percent for herbivores and carnivores, and perhaps 30 percent for secondary 
carnivores. Efficiencies for a particular species may be very much lower, 

TABLE 4-7 

Production and Respiration in Two Ecosystems (kcal/m2/yr) 

Field of alfalfa Rain forest 

Gross primary production (GPP) 24,400 45,000 

Autotrophic respiration 9,200 32,000 

Net primary production (NPP) 15,200 13,000 

Heterotrophic respiration 800 13,000 

Net community production (NCP) 14,400 essentially zero 

NPP/GPP (%) 62.3 28.9 

NCP/GPP (%) 59.0 

Field of alfalfa Rain forest 

*In the United States. Data from M. O. Thomas and G. R. Hill, 
Photosynthesis under field conditions. In Photosynthesis in plants, J. Franck 
and W. E. Loomis, eds., Iowa State College Press, Ames, 1949, pp. 19-52. 

Data from H. T. Odum and R. F. Pigeon, eds., A tropical rainforest: 
A study of irradiation and ecology at El Verde, Puerto Rico, National Technical Information 
Springfield, Va., 1970. 

Source: Adapted from E. P. Odum, Fundamentals of ecology. 

however. (Of course, care must be taken in calculating efficiencies in order to avoid, say, 
computing the ratio of dry dog food consumed to wet weight of dog produced!) 

It is important to keep in mind that basically these efficiencies are a consequence of the second 
law of thermodynamics — that in the respiratory process some of the high-grade energy of the 
chemical bonds is degraded to heat at a temperature so close to ambient that organisms have not 
been able to evolve a way to make it do work — in other words, it has become inaccessible to the 
ecosystem. It is this inexorable action of the second law that produces the pyramidal form of the 
diagrams in Figure 4-18 . The relationships between trophic levels are conventionally illustrated 
by pyramid diagrams based on numbers of individuals, energy, or biomass. 

Figure 4-19 shows pyramids of numbers for a grassland and for a temperate forest in summer. 
Note that the forest pyramid is partly inverted because of the large sizes of the individual trees. 
Figure 4-20 shows pyramids of biomass for a tropical forest community in Panama and the 
marine community of the waters of the English Channel. The numbers pyramids show the 
numbers of organisms that exist at an instant of time; the biomass pyramids show the dry weight 
of the crop of organisms at a given instant. 

Note that the English Channel pyramid is also inverted ~ the biomass of phytoplankton is 
smaller than that of zooplankton. Does this mean that plankton have 

40 T. S. Petipa, E. V. Pavlova, and G. N. Mirov, The food web structure: Utilization and 
transport of energy by trophic levels in the planktonic community, In Marine food chains, J. 
H. Steele, ed., University of California Press, Berkeley, 1970, pp. 142-167. 

41 D. G. Kozlowsky. A critical evaluation of the trophic level concept: pt. 1, Ecological 
efficiencies, Ecology, vol. 49 ( 1968), pp. 48-60. 



H -200, OCX) 



P— 1,500.000 

TC— 2 

C— 120.000 

h— 1 :^V--;i;;/:^vj«?5g?.'; 



FIGURE 4-19 

Pyramids of numbers for a grassland and a temperate forest community in summer. The numbers 

represent individuals per 1000 square meters; P = producers; H = herbivores; C = carnivores; TC 

= top carnivores. Microorganisms and soil animals are excluded. (Adapted from Odum, 1971.) 



D— 10 


H+C— 21 

P— 4 

FIGURE 4-20 
Pyramids of biomass for the English Channel marine and the Panamanian tropical forest 
communities. The numbers represent grams of dry weight per square meter; P = producers; H 
herbivores; C = carnivores; D = decomposers. (Adapted from Odum, 1971.) 

TC— 1.5 

C— 11 

H— 37 

P— 809 

D— 5 


C— 383 

H— 3368 

P— 20.810 

. ■ H ■•■*•■ '- *• ■ 1 " 1 

' f . ■ - - * • .«, £ , m I *.- 


FIGURE 4-21 
Standing crop and energy flow pyramids for an aquatic community at Silver Springs, Florida. P 
= producers; H = herbivores; C = carnivores; TC = top carnivores; D = decomposers. (Adapted 

from Odum, 1971.) 

found a way around the second law? Hardly ~ the pyramid of productivity for the English 
Channel community is right side up. The key point is that in that community the phytoplankton 
have a much more rapid turnover than the zooplankton. As an analogy, suppose you had on your 
kitchen shelf only one day's food at a time and the shelf was restocked every night. Since a 
biomass pyramid represents the situation at a given point in time (rather than productivity over 
time) a biomass pyramid of the shelf-person system would also be inverted. 

In general, small organisms use energy at higher rates (the metabolic rate) and reproduce more 
rapidly than large ones. Thus, for a given amount of energy flowing into a trophic level, the 
standing crop or biomass will vary directly with the size of the organisms. The biomass of 
elephants that could exist on a given chunk of Africa would be much greater than the biomass of 
a hypothetical insect population that ate precisely the same plants. 

The relationships among size, standing crop (biomass), and energy flow can be seen in Figure 4- 
21 . Note that the standing crop of decomposers (bacteria and fungi) is proportionally tiny — only 
0.58 percent of the biomass of the community. In contrast, they account for 17 percent of the 
energy flow in the community. In a sense, the energy- flow pyramids give the most accurate 
assessment of the roles of organisms at various trophic levels. Numbers pyramids tend to 
exaggerate the importance of small organisms; biomass pyramids, to underrate them. And, as 
long as the second law holds, energy- flow pyramids will show no inversions! 

It is, of course, extremely important to keep the difference between biomass and productivity in 
mind when considering the exploitation of a population. As one might expect, productivity is the 
critical variable. Knowledge of the total biomass of blue whales in existence is much less 
important in determining a 


rational harvesting scheme than knowledge of the weight of blue whales produced annually 
(there are many other important variables, as well — see Chapter 7). 


Biological communities are not static. They change over relatively long periods of time — 
hundreds, thousands, and millions of years — as the climate of Earth changes and as their 
component species evolve and coevolve. They also change on a time scale of years, decades, and 
centuries, in a process known as succession. Suppose a piece of land is newly exposed by the 
retreat of a glacier, the lowering of a lake, or the emergence of land from the ocean by volcanic 
action. Gradually the exposed rock is fragmented under the action of the wind, running water, 
and alternate heating and cooling, and soil starts to form. Plants such as lichens invade and speed 
the soil formation through chemical interactions with the substrate. Grasses and herbs then move 
in, adding the action of their roots (and their decaying remains) to the process. Herbivores arrive, 
followed by carnivores that feed upon them; then the process of primary succession is well 

Primary succession is the development of a biological community where none existed 
previously. The terrestrial process begins with bare substrate and, if undisturbed, continues until 
a relatively stable community characteristic of the climate regime and the soil type develops. 
Such a community is called a climax. The entire sequence of communities from bare ground to 
the climax is called a sere, and intermediate communities are called serai stages. Primary 
succession, beginning with a lake and progressing to a forest, is diagramed in Figure 4-22 . 

The earliest studies of succession were done around the turn of the century on a sere from bare 
sand substrate to a beech-maple climax forest on dunes around the shore of Lake Michigan. 
Pioneering ecologists H. H. C. Cowles and V. E. Shelford studied the plants and animals of this 
successional series, and the same system was reinvestigated by J. S. Olson a half-century later. — 
In the later study it was determined through carbon dating that the entire successional process 
took about 1000 years — a relatively long time. This historic sere was threatened by industrial 
development when one of us first visited it in 1958, and efforts were being made by 

conservationists to preserve the dunes as a recreational area and outdoor teaching facility. At this 
writing the issue remains unresolved, because although Congress set aside 3360 hectares of 
Indiana dunes as a national park, a nuclear power plant and adjacent industrial development are 
threatening it. 

Secondary succession occurs when a climax community is destroyed — say, by a fire or by 
human activities. It differs from primary succession largely in the early stages — the time- 
consuming process of soil-building is partially or wholly unnecessary. Human beings are often 
responsible for secondary succession, as when a clearing in a tropical forest is abandoned by a 
milpa farmer or when a dirt road or railway right-of-way is no longer used. 

A great deal has been written and many names have been coined by ecologists attempting to 
define and classify climaxes. The problem is fundamentally one of time — how long must a 
community last in a changing world in order to be considered a climax? No precise answer can 
be given. In periods of relatively little climatic change, a penultimate serai stage (the subclimax) 
may last a long time before an even more persistent climax stage replaces it. In parts of northern 
California, a grassland disclimax (a climax maintained by disturbance) is created by overgrazing 
in areas that otherwise would be live-oak woodland (cattle eat the young oak saplings, leaving 
the grassland that appears in Figure 4-23 ). 

There generally are significant changes in the ratio of gross production, P (or GPP), to total 
community respiration, R (autotrophic and heterotrophic as succession proceeds. ^_At the early 
stages of normal succession, the P/R ratio is greater than 1. Material is being added to the 
system, and net community production is high. As the climax P/R approaches 1, the system is 
more or less in equilibrium, and NCP is low or ( Figure 4-24 ). 

There are other differences, too. At the early stages of 

42 J. S. Olson, "Rates of succession and soil changes on southern Lake Michigan sand dunes", 

Botanical Gazette, vol. 119 ( 1958), pp. 125-170. 
43 A fine summary of the differences between early and late serai stages was given by E. P. 

Odum in The strategy of ecosystem development, Science, vol. 164 ( 1969), pp. 262-270. 
■135- PAGENUMBER136 



5^W,(1^./..Lm,';;] l 

"•."•■ l 

■'i'V'i' ■ 

Bgg^B » ?^ig^fcS » *.w> . ■/;.-.-.v;i 



- i 


— r-.-^ 

FIGURE 4-22 

Primary succession leading to the obliteration of a lake. The process starts at the edge of the 

water (top); where a few bog-adapted conifers rise in a forest of hardwoods. Next the debris of 

shallow-water plants turns the lake margin into marsh, which is gradually invaded by mosses and 

bog plants, bog-adapted bushes and trees such as blueberry and willow, and additional conifers. 

Eventually the lake, however deep, is entirely filled with silt from its tributaries and with plant 

debris. In the final stage (bottom) the last central bog grows up into forest. (Adapted from 

Powers and Robertson, 1966.) 

FIGURE 4-23 A grassland disdimax maintained by cattle grazing in the foothills of the Santa Cruz 

Mountains, central California, Notice the absence of oak seedlings. If the cattle were removed, this 

area would revert to oak woodland, (Photo by P. R. Ehrlich.) 

FIGURE 4-23 A grassland disclimax maintained by cattle grazing in the foothills of the Santa 

Cruz Mountains, central California. Notice the absence of oak seedlings. If the cattle were 

removed, this area would revert to oak woodland. (Photo by P. R. Ehrlich.) 


FIGURE 4-24 Succession in a forest. GPP = gross primary production; NCP net community 
Production; R = total community respiration; B = total biomass. (Adapted from Odum, 1971.) 

succession, food chains tend to be rather linear, and the bulk of the energy flows through the 
herbivores (through the so-called grazing food chain). In the climax the food chains tend to be 
woven into complex food webs (as discussed below and as much as 90 percent of the NPP 
bypasses herbivores and goes directly into the detritus food chain — that is, to the decomposers. 

It is useful to think of modern people as grazers -animals tied to the eating of grass — since the 
vast majority of human food is either grain or animals fed on grains or other grasses. It is not 
surprising, therefore, that a major human activity is keeping succession from proceeding beyond 

the early serai stages: in other words, in farming. Farms are man-made simplified ecosystems in 
which farmers attempt to maximize the P/R ratio. In this endeavor, considerable energy must be 
expended (weeding and eliminating competing herbivores) to prevent the natural processes of 
succession and to counteract other forces that tend to destroy what is an inherently unstable 

-137- PAGENUMBER138 

Complexity and Stability 

What tends to make early serai stages in general, and farms in particular, less permanent features 
in the landscape than, say, the oak-hickory forest climax? One explanation frequently cited is 
that farms are less complex. The climax contains more species of plants, animals, and 
microorganisms, and their food chains are more intricately interwoven. That there is a causal 
connection between complexity (or diversity) and stability (which in one sense is thought of as 
resistance to change in species composition and numbers of individuals) in biological 
communities is part of the folk wisdom of ecology. Six lines of evidence for this notion were 
presented by the famed British ecologist Charles Elton in his classic book, The ecology of 
invasions by animals and plants. 

Mathematical models of simple ecological systems indicate little stability; ordinarily there 

are great fluctuations in numbers of the population of each species. 

In simple laboratory systems, such as those containing a single predator species and a single 

prey species, extinction is the normal outcome for the predator or for both species. 

Natural communities on small islands are much more vulnerable to invasion by organisms 

that did not evolve with the community than are continental communities. This is especially 

true of oceanic islands with depauperate biotas (relatively few species present). 

Cultivated land (communities greatly simplified by human beings) is especially subject to 

invasions and to population explosions of pest species. 

Species-rich tropical rain forests are not as subject to insect outbreaks as are less-complex, 

temperate-zone forests. 

Pesticide treatment of orchards has resulted in pest outbreaks by upsetting the relationships 

between pests and their predators and parasites. 

These observations appear to be confirmed by many others. Recall that Heliconius ethilla in 
tropical forests is extremely stable in numbers, whereas populations of Euphydryas editha in 
temperate areas fluctuate greatly. In addition, the floristically rather uniform northern coniferous 
forest seems especially vulnerable to outbreaks of insect pests; and heavy use of pesticides has 
frequently created pest problems — for example, the famous Canete Valley disaster in Peru 
(described in Chapter 11). 

To understand one possible mechanism for the diversity-stability relationship, consider a 
complex food web (the intertwined food chains of a community). The food web of a Long Island 
estuary was thoroughly investigated by biologists George M. Woodwell, Charles F. Wurster, and 
Peter A. Isaacson. 44 Some of the relationships they discovered are represented in Figure 4-25 . 
Their study illustrates several important characteristics of most food webs, one being complexity. 
Although the figure shows only a sample of the kinds of plants and animals in this ecosystem, it 
is evident that most of the consumers feed on several different organisms and that most prey 
organisms are attacked by more than one predator. To put it another way, the food chains are 

interlinked. This interlinking may be one of the reasons that complexity is associated with 
stability in ecosystems. Presumably, the more food chains there are in an ecosystem and the 
more cross-connecting links there are among them, the more chances the ecosystem has to 
compensate for changes imposed upon it. 

For example, suppose the marsh-plant—cricket—redwing-blackbird section of Figure 4-25 
represented an isolated entire ecosystem. If that were the case, removing the blackbirds — say, by 
shooting — would lead to a plague of crickets. This in turn might lead to the defoliation of the 
plants, and then to the starvation of the crickets. In short, a change in one link of such a simple 
chain would have disastrous consequences for the entire ecosystem. Suppose, however, that the 
cormorants were removed from the larger system. Populations of flukes and eels would probably 
increase, which in turn might reduce the population of green algae (Cladophora). But there 
would be more food for mergansers and ospreys, so those populations would probably enlarge, 
leading to a reduction in the numbers of eels and flukes. In turn, the algae population would 

Needless to say, things do not normally happen that 

44 George M. Woodwell, "Toxic substances and ecological cycles", Scientific American, March 

Bottom O.J pound per acre 

, . ... 

Ma»h pMnti 

snort* □ 33 
Auk iac 

nmnmg bni*bi<4 Vv 

-» ei«M3VHjCW 

FIGURE 4-25 Portion of a food web in a Long Island estuary. The arrows indicate the flow of 
energy; the numbers tell how many parts per million of DDT are found in each kind of organism. 

(Adapted from Woodwell, 1967.) 

simply and neatly in nature. But there appear to be, then, both observational and theoretical 
reasons to believe that a general principle holds: stability is related to complexity. Complex 
communities such as the deciduous forests that cover much of the eastern United States persist 
year after year if people do not interfere with them. An oakhickory forest is quite stable in 
comparison with an ultrasimplified community such as a cornfield, which is a man-made stand 
of a single kind of grass. A cornfield has little natural stability and is subject to almost instant 
ruin if it is not constantly managed by the farmer. Similarly, arctic and subarctic ecosystems, 
which are also characterized by simplicity, tend to be less stable than complex tropical forest 
ecosystems. In arctic regions the instability is manifested in frequent, violent fluctuations in the 
sizes of populations of such northern animals as lemmings, hares, and foxes. In contrast, 

outbreaks of one species seldom occur in complex tropical forests. The late Robert MacArthur, 
who played a key role in stimulating the surge of interest in theoretical ecology in the last two 
decades, suggested in 1955 that the stability of an ecosystem is a function of the number of links 
in the web of its food chains. He developed a measure of that stability using information theory. 


More recent work, however, indicates that the relationship is more complicated and difficult to 
explain. For example, in some experimental work, an increase in diversity at one trophic level 
decreased stability at the next higher level; in other work it did not. ^_It turns out that both of the 
terms complexity (diversity) and stability have many meanings. Complexity, for instance, may 
refer merely to the species diversity — how many kinds of organisms are present and the 
equitability of their abundance. (A community consisting of 100 individuals of each of three 
species is more diverse than one having 10 individuals of each of two species and 280 
individuals of a third.) Or the term may be used to refer to the spatial diversity or patchiness of 
the environment (a number of vertical layers in the vegetation; uniform fields or fields 
interspersed with hedgerows). It also may include the genetic diversity within populations. 

Stability, as the term is most generally used, refers to the propensity of an ecosystem (or any 
system, for that matter) to return to equilibrium following perturbation. Since equilibria are 
difficult to define and measure in entire ecosystems, however, very often definitions of stability 
are based on the amplitude of fluctuations in the sizes of sample populations in an ecosystem. 

Recently the mathematics of complexity-stability theory has been synthesized by R. M. May, — 
who has shown convincingly that stability does not increase as a simple mathematical 
consequence of an increase in species diversity. But the mathematical models of ecosystems 
considered by May involve so many simplifying assumptions — especially the absence of 
coevolution — that they really do not address the key question of whether diversity causes 

Two other reviews of the diversity-stability question, by ecologists Daniel Goodman and 

William W. Murdoch, ^_both conclude that in natural ecosystems there is no reason to believe 
that diversity produces stability. Both cite numerous examples casting doubt on the classic 
Eltonian formulation of the relationship and the evidence that has been rallied to support that 
formulation. Murdoch focuses on the coevolutionary nature of species interactions in natural 
systems. He suggests, "The marked instability of agroecosystems (and other artificial 
communities), in contrast with the stability of natural communities, results from the frequent 
disruption of crops by humans and from the lack in crop systems of co-evolutionary links 
between the interacting species. This second feature of crops is caused by the haphazardness of 
the collection of species on any given crop field, the changing selective regime imposed by 
humans, and the fact that crops [crop ecosystems] have lost many species that were present in the 
previously existing natural communities." — 

Subsequently, however, Jonathan Roughgarden in a theoretical study of the role of coevolution 
in communities has shown that it may be either a stabilizing or a 

45 Fluctuations of animal populations and a measure of community stability. Ecology, vol 36, pp. 

46 L. E. Hurd, M. V. Mellinger, L. L. Wolf, S. J. McNaughton, "Stability and diversity at three 

trophic levels in terrestrial successional ecosystems", Science, vol. 173 ( 1971), pp. 1134- 

1 136; and C. A. Bulan and G. W. Barrett , The effects of two acute stresses on the arthropod 

component of an experimental grassland system, Ecology, vol. 52 ( 1971), pp. 597-605. 

Stability and complexity in model ecosystems . 

"The theory of diversity-stability relationships in ecology", Quarterly Review of Biology, vol. 

50 (1975), pp. 237-266. 
49 Diversity, complexity, stability and pest control. 
5Q Ibid„ p. 806 . 

destabilizing force. 51 In some circumstances coevolution can greatly enchance community 
stability, leading to a structure that will not collapse if a single species is removed. More 
frequently it leads to a configuration less stable than the original — to situations where, for 
example, the loss of a single species population may lead to the decay of the entire system. 

The last word on the relationship between diversity and stability is clearly not in. Perhaps the 
best hypothesis available today is that where one finds an association between diversity and 
stability it is probably the result of parallel evolutionary trends rather than a direct causal 


relationship. To put it another way, an increase in the complexity of ecological systems does 

not necessarily lead to an increase in their stability and, in fact, may destabilize them. But in the 
course of evolution the most stable complex systems are by definition those that have persisted 
for long periods. Unstable complex systems, on the other hand, have disappeared. 

Some tentative conclusions can be drawn from this. First of all, merely adding more species to 
agricultural ecosystems will not necessarily stabilize them. Indeed, it may have exactly the 

opposite effect. Van Emden and Williams cite numerous examples of this before giving a 

weak endorsement to the desirability of maintaining hedgerows between fields. They may take a 
somewhat too gloomy view of the benefits of such maintenance of spatial diversity, however, 
since they do not consider the positive effects of the hedgerows as pollinator refuges. But it is 
clear that careful evaluations of all the properties of the coevolved complexes of an ecosystem 
are necessary before attempting to tinker with increasing diversity in order to stabilize such 

On the other side of the coin, one can say that removing elements from relatively stable natural 
ecosystems may well destabilize them, with unfortunate consequences for Homo sapiens, which 
depends on those systems for a variety of indispensable services. For instance, in Africa a 
reduction in the diversity of the antelope fauna by brush clearing has destabilized the grazing 
ecosystem, and the game herds are diminishing. The productivity of herbivores in the new 
system, in which cattle are replacing the antelopes, is much less, as is the potential of that system 
for supporting human life. — 

Loss of herbivore productivity is not the only possible consequence of reducing complexity. 
Ecologist George Woodwell has emphasized the tightness of normally complex, mature 

ecosystems. Several studies show that disturbance (for example, cutting down the trees of a 
forest) leads to a leakage of nutrients from a system. Such leakage, according to Woodwell, 
"may be large enough under certain circumstances to reduce the potential of the site for repair 
through succession and to degrade other systems by overloading them with nutrients." ^_The 
phenomenon of nutrient leakage is worldwide, but it seems especially critical in that most 
threatened of all ecosystems, the tropical rain forest. 

Unhappily, there is no way to predict exactly what the consequences will be of removing even 
one species from an ecosystem or even of reducing the genetic variability of a species. In some 
cases a removal may make no discernable difference; in others it may be catastrophic. In one 
instance, removal of a single species from a fifteen-species intertidal community of marine 
invertebrates caused it to collapse to an eight-species system in less than two years. ^Similarly, 
it is clear from the work in lupines and lycaenid butterflies discussed earlier that genetic diversity 
may be very important in the trophic relationships of plant populations. 

Diversity, stability, and ecological politics. Ecologists have commonly argued for the 
preservation of diversity because they have believed that the stability of ecological systems may 
depend upon it. Now, as we have seen, considerable doubt has been thrown on the notion 

51 Coevolution in ecological systems, part 2: Results from "loop analysis" for purely density- 
dependent coevolution, in Symposium on measuring natural selection in natural populations, 
F. Christiansen and T. Fenchel , eds., Springer-Verlag, Aarhus, in press. 


R. Margalef, "Diversity and stability: A practical proposal and a model of interdependence", 

in Diversity and stability in ecological systems, USAEC, Washington, D.C., Brookhaven 

Symposia in Biology, No-vember 22, 1969. 
""Insect stability and diversity in agroecosystems", Annual Review of Entomology, vol. 19 ( 

1974), pp. 455-475. 
54 W. H. Pearsall, The conservation of African plains game as a form of land use, in The 

exploitation of natural animal populations , E. D. LeCren and M. W. Holdgate, eds., Symposia 

of the British Ecological Society, Blackwell, Oxford, 1962, pp. 343-383. 
55 Success, succession, and Adam Smith, BioScience, vol. 24, pp. 81-87. 
56 R. T. Paine, Food web complexity and species diversity, American Naturalist, vol. 100 ( 

1966), pp. 65-75. 

that diversity causes stability, and even on the idea that the two are associated. It therefore is 
important that general statements about diversity, complexity, and stability now be eliminated 
from the political rhetoric of the environmental movement, for that movement must give as 
accurate an interpretation of the current state of ecological science as possible. 

It is important to note that the diversity-causes-stability argument is not necessary in order to 
demonstrate that extermination of populations or species, destruction of genetic variability, and 
other actions that simplify ecological systems are threatening not just to those systems but to 
humanity as well. Lessening the diversity of both simple and complex natural ecosystems may 
lead to their destabilization, with results that are extremely undesirable from a human viewpoint. 

As Goodman succinctly put it, "From a practical standpoint, the diversity-stability hypothesis is 
not really necessary; even if the hypothesis is completely false it remains logically possible — 
and, on the best available evidence, very likely — that disruption of the patterns of evolved 
interaction in natural communities will have untoward, and occasionally catastrophic, 
consequences." — 

Extinction, Endangered Species, and Invasions 

It should be apparent from the preceding that humanity forces populations and species to 
extinction at its peril. Of course, extinction is a natural phenomenon -organisms have been going 
extinct for billions of years. There have been episodes of "rapid" extinction in geological history, 
such as the disappearance of the dinosaurs about 70 million years ago. Those rapid episodes, 
however, were actually quite slow compared to the extinction "explosion" now occurring. 
Records of the disappearance of species of birds and mammals have been reasonably accurate 
since 1600. Since then some 130 species have become extinct ~ roughly I percent of the 12,910 


species of birds and mammals alive 375 years ago. — 

Although past rates of extinction are very difficult to estimate, if the average "life span" of a 
species of higher vertebrate is assumed to be between 200,000 and 2 million years, then 
extinction of I percent of the species could be expected every 2000 to 20,000 years. If so, in the 
period from 1600 to 1975 the rate of extinction was five to fifty times as high as in the past. 
Furthermore, some 300 additional species of birds and mammals are now threatened with 
extinction. Should one-fifth of those actually become extinct before the year 2000, the rate would 
be from 40 to 400 times normal. Such rates of extinction are much too rapid for the normal 
evolutionary processes that generate organic diversity to be able to replace them. 

Human beings first became a significant force for extinction when they became hunters. There is 
little question that the extinction of many large mammal species during the late Pleistocene 
10,000 to 12,000 years ago was accelerated by the human search for food. 59 In more recent 
times, however, this has declined in importance as a cause, and the destruction of the habitats of 
birds and mammals has become a more important element in extinction. 60, For other animals and 
plants, habitat destruction is the primary cause of extinction. When those much more numerous 
organisms are added to the bird-mammal picture, the rate of extinction then seems to be about 
10,000 species per century (from a total of perhaps 10 million). If the Brazilians and others 
succeed in destroying the flora and fauna of the Amazon basin (as discussed in Chapter 11), 
perhaps 1 million kinds of plants and animals could disappear from that region alone. — 

Furthermore, in many (if not most) situations, saving small patches of habitat is not enough to 
prevent a decay of species diversity. A preserved area must be large enough to support all of the 
populations belonging there — and some mammalian carnivores and raptorial birds require 
territories of many square kilometers. Even populations of insects may require sizable areas for 
their maintenance. It seems likely, for example, that because of its trap-lining behavior a 
population of the butterfly 

The theory of diversity-stability, p. 261. See also Gordon H. Orians, Diversity, stability and 


maturity in natural ecosystems. 

Fisher, Simon, and Vincent, Wildlife in danger. 

P. S. Martin a 

Haven, 1967. 

59 P. S. Martin and H. E. Wright (eds.), Pleistocene extinctions, Yale University Press, New 

60 Vetz and D. L. Johnson, "Breaking the web", Environment, vol. 16, no. 10 ( December 1974). 
61 "Scientists talk of need for conservation and an ethic of biotic diversity to slow species 
extinction", Science, vol. 184 ( May 10, 1974), pp. 646-647. 
-142- PAGENUMBER143 

Heliconius ethilla requires at least one square kilometer. Many euglossine bees also require large 
areas of forest, and many bee populations and species are being wiped out as the forests of 
Central America are decimated. 62 Their loss, in turn, is having dire consequences for the orchids 
and other plants that depend on them for pollination. The future well-being of the ecological 
systems that support our civilization may be threatened by the extinction of these species. Some 
of the species now endangered may be key species in systems important to people, but there is no 
way to tell whether they are, without the most exhaustive investigations of the ecosystems 
involved. Who, for instance, would have guessed that E. J. Kuenzler would discover in his study 
of a Georgian salt marsh ^_that a seemingly insignificant mussel was largely responsible for 
making phosphorus (a limiting resource) available to the community? 

Thus, conservationists have a powerful argument for the protection of endangered species 
beyond the compassionate-esthetic-recreational arguments usually raised, although those are 
compelling in themselves. As far as anyone knows at this writing, the other species of organisms 
on Earth are our only living companions in the universe. Each one that is forced to extinction is 
one that our descendants will not have the opportunity to see (or exploit), just as we have no 
possibility of seeing alive dodos, passenger pigeons, or the Xerces butterfly. And, of course, each 
extinction carries with it the threat of the loss of other values as a pool of genetic information is 
destroyed forever. These losses range from potential sources of antibiotics and laboratory 
animals for medical research to genetic material useful for domestication, or, through crossing, 
for improving plants and animals already domesticated. 

Nor is it possible to insure adequately against such loss by maintaining samples of organic 
diversity in zoos, experiment stations, and botanical gardens. The problems of maintaining many 
species in captivity are extremely complex — and, as the numbers that need to be saved grow, the 
task could become immense. ^_But, even if culture techniques were perfected for most species, 
the serious problem of the decay of genetic variability would remain. 

Finally, it is important to remember that human beings decrease organic diversity, not only by 
directly assaulting species and by destroying habitats, but also by moving organisms around. 
Species transplanted are often species removed from the presence of natural enemies that keep 
their numbers in check, and inserted into the presence of others that have never evolved defenses 
against the invaders or the capacity to compete with them. In 1975 on the island of Maui in 
Hawaii, we were surprised to find a dense population of swallowtail butterflies. The only large 
butterflies on the Hawaiian islands previously had been monarchs. The new butterfly was Papilio 
xuthus, a common Asian species in a group that feeds on plants of the Citrus family Rutaceae. A 
phone call to the Hawaii Department of Agriculture revealed that P. xuthus had been in the 

Hawaiian islands for more than a year and on Maui for "about three months." In that time a 
population much denser than most Papilio populations had built up, apparently in the log phase 
of growth, and considerable damage was visible on the citrus trees, grown largely as ornamental 

What impact this import will have on native Hawaiian Rutaceae remains to be seen. The 
Hawaiian Islands, however, have lost a great deal of their native forests to clearing for 
agriculture and for urban and resort development. The unique fauna of Hawaiian honeycreepers 
(finchlike birds of the family Drepanididae) has already suffered severe losses. Eight of twenty- 
two species discovered before 1900 are now extinct or presumed extinct, and several others have 
lost distinct races from various islands. Forest clearing, the introduction of mongooses for rat 
control, and the introduction of foreign birds that both competed with the honeycreepers and 
carried disease, all played parts in this disaster. (The mongoose, by the way, is one of the most 
destructive exotic animals. Wherever it has been introduced to control rats or snakes, it has made 
haste to devour part of the local bird fauna as well.) 

Enormous areas may be dramatically changed by introduction of a single species freed of its 
natural controls. The flora of much of the Mediterranean area 

62 D. H. Janzen, "The deflowering of Central America", Natural History, April 1974. 
63 Structure and energy flow of a mussel population in a Georgian salt marsh, Limnology and 

Oceanography, vol. 6 ( 1961), pp. 191-204. 
64 David R. Zimmerman, "Captive breeding: Boon or boondoggle", Natural History, December 
1974, pp. 6-19. 

, *^w 





Temperaie tows! *nd ran torwl grassland 



T topical ram forest 

Troptc ai deciduous ior«i 

Tropica! acrub tot** 

Tropica] savanna and grassland 

Mo<*H*ir» (cwnplfti fontfwnl 

(ce cap 









15 10 5 


FIGURE 4-26 A. Distribution of the major biomes of the world. (Adapted from Odum, 1971.) B. 
The relationship of biomes to precipitation and temperature. The broken line encloses the range 

of values within which either grasses or woody plants may dominate, depending on local 
conditions. The figure is simplified, and the positions of the divisions are approximate. (Adapted 

from Whittaker, 1970.) 


today is impoverished because of the activities of that ultraefficient grazer and browser, the goat. 
Opuntia cactus imported into Australia covered vast areas until its populations were brought 
under control by a moth, Cactoblastis cactorum, taken there for the purpose from the native 
haunts of the cactus in South America. 

The history of human transfers of organisms, purposeful or accidental, is replete with disasters — 
the cabbage butterfly, the Hessian fly, the gypsy moth, the starling, and the walking catfish have 
made trouble in North America; rabbits overran Australia; and the grape pest Phylloxera nearly 
destroyed France's wine industry in the nineteenth century. The great ecologist and 
conservationist Aldo Leopold once said, "The first rule of intelligent tinkering is to save all the 
parts." Perhaps we should now add to that, "and don't put them in the wrong place." 


Terrestrial communities differ dramatically from place to place. The assemblage of plants and 
animals observed by a resident of California or Chile, for example, is strikingly different from 
that familiar to a resident of New Jersey or Great Britain. The major kinds of terrestrial 
communities are called biomes by ecologists. Since the impact of human activities varies greatly 
from biome to biome, you should be familiar with the distribution and at least some of the 
characteristics of these broadest groupings of communities ( Figure 4-26 A ). — 

Climatic factors exert the primary control over the nature of the biota (a collective term for the 
organisms) of 

65 Further details on the fauna and flora of biomes can be found in the books by Kendeigh; 
Odum; Raven, Evert, and Curtis; and Alice et aL., listed at the end of the chapter. 
-145- PAGENUMBER146 

TABLE 4-8 Biological Productivity in Differing Biomes 

Arctic Beech Subtropical Tropical 

tundra Northern forest deciduous rain Dry Desert 

( taiga (central forest forest savanna ( 

Biomass USSR) ( USSR) Europe) (average) (average) ( India) USSR) 


8 13 8 11 13 

70 73 77 













Photosynthetic parts 
(%) (mostly leaves) 

Perennial parts (%) 
(stems, etc.) 

Roots (%) / 70 22 26 20 

Total 100 100 100 100 

Litter fall (% of plant on 4 3 s 


Source: Modified from L. E. Rodin and N. I. Bazilevich, Production and mineral cycling in 

terrestrial vegetation. 

broad geographic areas — they are responsible for the characteristic life forms that permit 
ecologists to distinguish biomes. The relationship of average annual precipitation, average 
annual temperature, and biome type is shown in Figure 4-26B . Some of the characteristics of 
ecosystem productivity in the various biomes are listed in Table 4-6 . Table 4-8 p resents a picture 
of the distribution of biomass among roots and above-ground parts of plants and indicates what 
percentage of the biomass the litter fall is, in seven biomes. There are characteristics of the 
ecosystem in various biomes that transcend the collection of species functioning in the 
ecosystems. Thus, two tropical rain forests may have entirely different floras, but they will be 
much more similar to each other in their productivity and in the apportionment of biomass 
between roots and above-ground parts than either would be to a temperate forest or a desert 

Temperate Forest 

In regions where the minimum temperature is below freezing each winter but usually does not 
drop below — 12° C (10° F) and the annual rainfall is between about 75 and 200 centimeters (30- 
80 inches), the dominant plants and animals belong to the temperate forest biome. This is the 
biome familiar to most readers of this book, because it is the biome in which Western (and 
Chinese) civilization developed. It is typified by the deciduous forest of the eastern United States 
but is also found in Western Europe and Northeastern China. Where soil conditions permit, 
mixtures of broad-leafed deciduous trees, such as maples, hickories, and many oaks, grow in 
dense stands. In the American forests, characteristic mammals such as white-tailed deer, 

squirrels, shrews, bears, and raccoons forage; so do birds such as warblers, woodpeckers, a wide 
variety of thrushes and flycatchers, wild turkeys, and owls and hawks. 

The deciduous nature of the dominant trees plays a critical role in determining the nature of the 
temperate forest biome ( Figure 4-27 ) . It permits a rich secondary flora to coexist under the 
trees, flowering in the early spring before the leafing of the trees shades the flora out. In turn, the 
members of this secondary flora are producers of major components of the temperate forest food 
web. Many forest insects, for example, feed on these plants of the understory. Furthermore, the 
deciduous habit itself can be considered an evolutionary response of basically tropical flowering 
plants that permits them to survive periods when low temperatures would interfere with their 
photosynthetic mechanisms and water balances. 

Because of the annual leaf drop, temperate forests generate soils rich in nutrients, which in turn 
support an extravagant microbiota in the soil itself. A gram of forest soil has been found to 
contain 650,000 algae, and counts of microarthropods may number in the hundreds of thousands 
and nematodes (roundworms) in the millions per square meter. ^_Hence, soil is not just small 

66 L. Steubing, Soil flora: Studies in the number and activity of microorganisms in woodland 
soils, in Analysis of temperate ecosystems, David E. Reichle, ed. 
-146- PAGENUMBER147 

worn from the rock of Earth's crust, but a complex integral part of the ecosystem, containing 
both plants and animals. Especially important to the fertility of the soil is its content of 
decomposed organic matter or humus (see Chapters 7 and 11). 

When a temperate forest is cleared, the stored energy and nutrients in the trees are removed, as is 
the protection from wind and torrential rain that a forest canopy provides. Furthermore, an 
important source of new nutrients is lost, for the forest trees serve as nutrient pumps. Their deep 
roots bring vital substances up from the lower regions of the soil, and the trees deposit them at 
the surface when they shed their leaves. 

FIGURE 4-27 A deciduous forest. (Photo by George Taylor, courtesy Friends of the Earth.) 

When temperate forests are cleared, the richness of the soils can be preserved if great care is 
taken to see that their supplies of nutrients and decaying organic material are maintained. It is 
critical to remember that raising crops or grazing animals on land is, in a sense, mining the soil 
unless nutrients are carefully returned to it. The nutrients depart along with the crops, meat, 
wool, or whatever is removed. These may be returned in a variety of ways so the crucial nutrient 
cycles are continued (see Chapter 3). But, all too often, short-term careless exploiters have 
permitted soils to deteriorate or have ignored opportunities to improve them. Numerous 
abandoned farms in Virginia and the Carolinas show the cost to humanity of ignoring the 
fundamentals of soil ecology; the rich farms of, say, Bavaria show the rewards of paying careful 
attention to them. 

Unfortunately, economic considerations often lead individuals who are exploiting an ecological 
system to use short-term strategies that are disastrous for humanity in the long run. Thus, it might 
have been uneconomical for nineteenth-century farmers in some parts of the eastern United 
States to maintain the nutrient supplies in the soil of their farms carefully. Perhaps they could 
only survive by living on the capital of nutrients in the land they cleared. In more densely 
crowded Europe, without competition from other farmers continually exploiting an expanding 
frontier, a Bavarian farmer of the same period could afford the appropriate soil husbandry — a 
husbandry that was part and parcel of ancient local tradition. 


In areas where rainfall is less than 25 centimeters (10 inches) a year are found the world's 
deserts. They are concentrated in the vicinity of the latitudes 30° north and 30° south, where the 
global weather system tends to produce descending masses of dry air (although special local 
conditions may create deserts far from these latitudes). Lack of moisture is the essential factor 
shaping the desert biome. Most deserts are quite hot in the daytime and, because of the sparse 
vegetation and resultant rapid reradiation of heat, quite cold at night ( Figure 4-28 ). Some, like 
the fog desert of the Peruvian coast, which 

-147- PAGENUMBER148 

FIGURE 4-28 A desert plant community, central Baja California, Mexico. Notice the variety of 
spiny, succulent plants and their wide spacing. (Photo courtesy of Robert Ricklefs.) 

FIGURE 4-29 A fog desert on the Peruvian coast south of Lima (the shore is in the background). 
Note the lack of vegetation in this habitat. (Photo by P. R. Ehrlich.) 

FIGURE 4-30 An Arctic rock desert, Cornwallis Island, Northwest Territories, Canada. Like the 

fog desert, this habitat is often nearly devoid of plants, which in this area are most abundant on 

the relatively nutrient-rich sites of the middens of ancient Eskimo camps. (Photo by P. R. 


gets little sun over much of the year ( Figure 4-29 ) and the arctic rock desert ( Figure 4-30 ) are 
relatively cool even at midday. Both of these desert habitats can be nearly devoid of plants. 

Desert plants and animals have evolved many specializations for conserving water. In plants 
these include unusually thick, waterproof outer layers or cuticle, modifications of breathing pores 
(stomata), reductions in leaf area, and specialized hairs or outgrowths that reflect light. Many 
desert plants also are heavily armed with spines to repel the attacks of moisture-seeking animals. 
They frequently are aromatic, indicating the presence of biochemical defenses against 

Plants may appear to be widely spaced in deserts, but if their roots were visible, the ground 
between the plants would be seen to be laced with shallow root systems that allow maximum 
absorption of the rain that does fall. In certain soil conditions, desert plants may have extremely 
long taproots to reach deep underground water supplies. 

Germination of the seeds of desert plants is often inhibited by water-soluble chemicals that must 
be leached out by a threshold amount of rain before sprouting can occur. Other plants that grow 
in gravelly arroyos have seeds that require abrasion as the first rains wash them along before they 
will germinate. Thus, deserts appear to turn miraculously green almost overnight following 
significant rains, and fast-growing annual plants often create spectacular floral displays. 

Proportionately more annual plants are found in the desert biome than in any other. If rainfall is 
relatively abundant, each individual plant may produce numerous flowers. If rainfall is sparse, 
many individuals produce only a single blossom each. Since the period of bloom is brief and 
only follows sporadic rainfall, many people who have traveled in desert areas have not seen them 
in bloom and remain unaware of the wonderful aesthetic resource they represent. 

Desert animals also solve the problem of water shortage in diverse ways. Most are active 
primarily at night, remaining under cover in the heat of the day. Excretory systems are designed 
to conserve water, and many desert animals are able to use the water they produce in their 
cellular metabolism. Many desert insects are "annuals," like the plants they feed on — 
synchronizing their periods of activity with the evanescent desert bloom. 

Desert soils contain little organic matter and must ordinarily be supplied with both water and 
nitrogen if they are to be cultivated. Much of the uncultivated flat land remaining in the world 
today is desert, and it seems inevitable that large-scale attempts will be made to expand 
agriculture into this biome. As we discuss in Chapter 11, irrigation is an expensive and often 
temporary process — very often a system cannot be maintained and once-cultivated and lands 
revert to desert. 

Human activities already have produced a great increase in the amount of desert and wasteland. 
In 1882 land classified as either desert or wasteland amounted to 9.4 percent of the total land on 
Earth. In 1952 that proportion had risen to 23.3 percent. (Part of this increase doubtless is the 
result of better information or changes in the definition of wasteland.) During the same period, 
land classified as carrying inaccessible forest decreased from 43.9 percent to 21.1 percent. 67 The 
vast Sahara itself is in part man-made, the result of overgrazing, faulty irrigation, and 
deforestation, combined with natural climatic changes. Today the Sahara seems to be advancing 
southward into the drought-stricken Sahel, its advance aided by overpopulation of people and 
domestic animals. The great Thar desert of western India is also partly the result of human 

influence. Some 2000 years ago, what is now the center of that desert was a jungle. The spread 
of the desert has been aggravated by poor cultivation practices, lumbering, and overgrazing. 
Human activities could lead to repetition of the Sahara and Thar stories in many parts of the 
globe. - 


Where broad climatic conditions are intermediate between those favoring temperate forest and 
those producing desert, microclimatic factors (controlled by slope, exposure, and the like) 
determine whether forests grow. 

67 R. R. Doane, World balance sheet, Harper, New York, 1957. 

68 The Sahara and Thar situations are described in M. Kassas, Desertification versus potential 
for recovery in circum-saharan territories, in Arid lands in transition, Harold E. Dregne, ed., 
American Association for the Advancement of Science, Washington, D.C., 1970, and B. R. 
Seshachar , Problems of environment in India, in Proceedings of joint colloquium on 
international environmental science. U.S. Government Printing Office, Washington, D.C., 
1971, Report 63-562. 
-149- PAGENUMBER150 

Thus, tongues of forest may intrude into areas of desert climate along river valleys. The trees 
themselves change the microclimate in their vicinity, and if they are removed from an area 
otherwise suitable for their growth, a grassland may develop ( Figure 4-23 ). Many ecologists 
feel that the great grasslands of the American prairie and Great Plains, the steppes of Russia, the 
vast savannas of Africa, and so forth are all zones between forest and desert where fire and/or 
browsing animals have prevented the spread of trees, even though rainfall is more plentiful than 
that which produces typical desert. Various specializations of grasses lead to their dominance in 
such situations. 

Large animals typical of grasslands include the bison ("buffalo") and pronghorn antelope in 
North America; wild horses in Eurasia; large kangaroos in Australia; and zebras, giraffes, white 
rhinoceros, and a vast diversity of antelopes in the African savannas. The latter support the 
richest fauna of large grazers, and the grazers seem to have coevolved in ways that maximize the 
utilization of the local plant resources. — 

Other important grassland animals include lions, cheetahs, hyenas, coyotes, and other predators; 
a variety of birds, ranging in size from ostriches and vultures to small sparrows; rabbits and other 
burrowing rodents (including the prairie dog); and grasshoppers (locusts). Some grasshoppers 
exist in a solitary form when their population density is low but transform under crowded 
conditions over a generation or so into a gregarious form that differs strikingly in both 
appearance and behavior. The crowded forms migrate in huge masses, laying waste to the 
countryside — a plague of locusts. For a long time the origins of the locust plagues were not 
understood because the solitary and gregarious forms were so dissimilar they were identified as 
different species. — 

The grassland biome has a higher concentration of organic matter in its soil than any other 
biome. The amount of humus in grassland soil is about a dozen times that in forest soils. The 
extraordinary richness of grassland soil has led to the establishment of extremely successful 
agricultural ecosystems in grassland areas — in the American prairies, for example. These 
agricultural systems can deteriorate rapidly, however, if careful soil husbandry is not practiced. 

The interlaced roots and creeping underground stems of grasses form a turf that prevents the 
erosion of soil by wind and water. When the turf is broken with a plow, however, the soil is 
exposed to those erosive influences. In addition, certain kinds of plowing can lead to the 
formation of a hardpan (a soil horizon nearly impervious to water and roots) below the surface. 
These two factors have led to rapid deterioration of agroecosystems established in some 
grasslands — a deterioration dramatized by the dust bowl of the American Great Plains in the 
1930s, described so graphically by John Steinbeck in his classic The Grapes of Wrath. Indeed, 
lack of proper soil conservation has already led to a loss of an estimated one-third of the topsoil 
of the United States (see Figure 4-31 ). — 

The situation elsewhere is even more serious. It is estimated that half the farmland in India is not 
adequately protected from erosion, and on fully one-third of the farmland, erosion threatens to 
remove the topsoil completely. As agronomist Georg Borgstrom has pointed out, soil 
conservation procedures are especially difficult to institute in areas where a population is poorly 
fed. He cites a study that recommended a one-fifth reduction in the amount of cultivated land and 

a one-third reduction in the size of livestock herds in Turkey. It was hoped that these 

reductions would help to diminish the danger of catastrophic erosion caused by overgrazing. 
Unfortunately, the program was not initiated, presumably because the local people depended 
upon the land and the herds for food and other necessities. As so often happens, a short-run need 
took precedence over long-run wisdom. 

Agriculture in grassland biomes can only be successful over the long term in areas where 
considerable effort is put into the maintenance of soil structure and nutrients. As we discuss in 
Chapters 6 and 1 1 , there is considerable reason on this account for anxiety about the prospects 
for continued high productivity in the American Midwest. 

69 See, for example, S. J. McNaughton, "Serengeti migratory wildebeest: Facilitation of energy 


flow by grazing", Science, vol. 191 ( January 9, 1976), pp. 92-94. 

See V. B. Wigglesworth, The life of insects, New American Library, New York, 1968, or any 

standard entomology text. 

71 Kendeigh, Ecology. 
12 Too many, Georg Borgstrom, Macmillan, New York, 1969. 


FIGURE 4-31 A. A dust storm approaching Springfield, Colorado, May 21, 1937. The storm 

caused a half hour of total darkness in the late afternoon. B. A farm abandoned in Bacca County, 

Colorado, after wind had blasted topsoil from fields and moved it over the farmhouse and other 

structures. The plow in the foreground was left at the end of a row in the field. (Photos courtesy 

of the U.S. Department of Agriculture, Soil Conservation Service.) 


North of the temperate forests in the Northern Hemisphere lies a broad zone of coniferous forest, 
generally called by its Russian name, taiga ( Figure 4-3 2 A ). Most of the trees do not shed all 
their leaves in winter. Their leaves are specialized to reduce water loss, especially in the cold 
season when the area is, in physiological effect, a desert. The leaves in many cases are needlelike 
and generally last three to five years. Although one thinks of spruce, fir, and other conifers as 
typical of the taiga, in local regions — including disturbed areas —such deciduous trees as aspens, 
alders, and larches may be prominent elements of the flora ( Figure 4-32B ). In general, the trees 
are much less diverse than those in temperate forests, and the soils have a different kind of 
humus and are more acid. 

Bears, moose, lynxes, rabbits, squirrels, and a variety of birds live in the taiga, but the diversity 
and abundance of warm-blooded vertebrates are generally less than those of the temperate 

forests. An exception to this rule are the mustelids (weasels, martins, sables, fishers, and 
wolverines), which are relatively richly represented. The diversity of cold-blooded vertebrates is 
even more dramatically restricted — snakes are uncommon, and few amphibia are to be found. 
Insect diversity is correspondingly low, but the existence of huge stands of one or two species of 
conifer provides an opportunity for periodic 

v^**. . . 

A~. I 

FIGURE 4-32 A. Taiga along the Hay River, Northwest Territories, Canada. A growth of aspens 

and willows occupies the area subject to flooding along the riverbank. (Photo by P. R. Ehrlich .) 

B. Taiga in winter northeast of Fort Yukon, Alaska (north of the Arctic Circle). This picture 

shows characteristic soil polygons, formed through expansion and contraction during freezing 

and thawing. (Photo courtesy of the U.S. Department of Agriculture, Soil Conservation Service.) 

C. Taiga in summer near Great Slave Lake, Northwest Territories, Canada (south of the Arctic 

Circle), showing large areas of muskeg (marsh) and numerous small lakes characteristic of much 

of the Canadian subarctic. (Photo by P. R. Ehrlich). D. Mosquito protection is required on the 

Great Slave Lake, Hay River, Northwest Territories, Canada. Note the mosquitoes on the pith 

helmet. (Photo by P. R. Ehrlich.) 

FIGURE 4-33 Nothofagus, the southern beech, on the shores of Lake Fagnano, Tierra del Fuego, 
Argentina. The slopes of the far shore are covered with Nothofagus forest. (Photo by P. R. 

Ehrlich .) 

outbreaks of herbivores like the spruce budworm (the larva of a moth, Choristoneura 
fumiferana), which can defoliate huge areas of forest. In the early 1950s in New Brunswick, 

Canada, more than 6 million acres were defoliated by the spruce budworm. Mosquitoes and 

other biting flies reach abundances in the taiga unknown elsewhere in the world and can make 
human life very difficult in summer ( Figure 4-32D ). The uniformity of the taiga ecosystem in 
comparison with the temperate forest and its comparative sensitivity to insect plagues has been 
one of the underpinnings of the notion that diversity in ecological systems tends to be associated 
with their stability. 

First animal pelts for the fur trade and then wood were the major commodities people extracted 
from the taiga. Decimation of the fur-bearing animals, changes in clothing styles, and a growing 
demand for pulp for manufacturing paper have led to an almost total shift to logging in the 

exploitation of this system. Huge areas have been denuded of forest, often with little or no 
attempt to protect the soil, so in many places the existence of the climax coniferous forests of the 
taiga is threatened by erosion. Furthermore, broadcast use of insecticides against insects has been 
widespread, adding persistent poisons to the already heavily taxed ecosystems of the planet and 
normally failing to fulfill the goals of the spray program. — 

In the Southern Hemisphere, little of the land area extends to high enough latitudes to support an 
extensive taiga like that of the Northern Hemisphere. A somewhat comparable ecosystem is 
found in the cool areas of South America and Australia, dominated by evergreen trees of the 
beech family in the genus Nothofagus ( Figure 4-33 ). A characteristic of Nothofagus forests, like 
those of the taiga, is an extensive litter of tree trunks and branches on the forest floor, since in 
cold climates the action of decomposer organisms is quite slow. 


North of the taiga lie the treeless plains of the tundra. In those areas, where the average annual 
temperature is below -5° C (23° F), the ground normally thaws in summer only to a depth of less 
than a meter. Below that it remains permanently frozen (permafrost). Precipitation is usually less 
than 25 centimeters (10 inches) per year. The permafrost effectively prevents the establishment 

73 R. F. Morris, (ed.), The dynamics of epidemic spruce budworm populations, Memoirs: of the 

Entomological Society of Canada, vol. 31 ( 1963) pp. 1-332. 
74 Ibid. A lso, see Chapter 11 for a fuller discussion of the use of insecticide. 
■153- PAGENUMBER154 

FIGURE 4-34 Tundra biome. A. Grassy tundra at Duke of York Bay, Southampton Island, 

Northwest Territories, Canada. The bones in the foreground are the vertebrae of a right whale at 

the site of an old Eskimo encampment. The right whale is now extinct in this area. B. An Eskimo 

child holding a piece of white whale (beluga) skin on Coral Harbour, Southampton Island, 1953. 

The skin of this small whale is considered such a delicacy that the Eskimo word for it, muktuk, 

forms the root of the Eskimo word for "delicious," a nah muktuk. The tundra biome in most areas 

is relatively devoid of large food animals, the exception being migratory caribou. Therefore, 

primitive Eskimo groups were heavily dependent upon sea mammals and fishes for food and 

other resources. C. Eskimo men butchering walruses on Walrus Island in northern Hudson Bay, 

Canada. Walrus provide food for sled dogs, skin for making harness, and ivory for carving. D. 

Low tundra with extensive willow stands around lakes on Southampton Island. (Photos by P. R. 

Ehrlich.) E. Tundra below Mount McKinley, Alaska, showing caribou trails. (Photo by Charlie 

Ott, courtesy National Audubon Society.) 
-154- PAGENUMBER155 

trees. Not surprisingly, the soil fauna is poorly developed — nearly as depauperate as that of 

deserts. Lichens, sedges, grasses, willows (which normally reach a height of less than a meter), 

and members of the cranberry family are among the dominant plants ( Figure 4-34A ). In some 
areas of the central Canadian Arctic, however, rainfall is so low that a rock desert nearly devoid 
of vegetation occurs ( Figure 4-30 ). 

Caribou, wolves, musk-oxen, arctic foxes, rabbits, and (occasionally in summer) polar bears are 
among the mammals found in the tundra. In the brief arctic summer, an astounding diversity of 
birds (especially waterfowl), migrate to the tundra to breed, feeding on an ephemeral bloom of 
insects and freshwater invertebrates. Mosquitoes can make one's life nearly as miserable in the 
tundra as in the taiga. Reptiles and amphibians are absent. As in the taiga, animal populations in 
the tundra may be subject to dramatic oscillations in size. Best known are the lemming cycles, 
which induce cycles in the owls, jaegers (predatory gulls), and other predators that feed on them. 

Because of the slow rates at which plants grow and decompose, with the low temperatures and 
the characteristics of permafrost, the thick, spongy matting of lichens, grasses, and sedges that 
characterizes the "low tundra" (that found in depressions and on plains rather than ridges and 
slopes) is especially slow to recover from disturbance. 76 Tracks of vehicles or animals can 
remain visible for decades ( Figure 4-34E ). Great care must be taken in building on tundra 
because heat from structures will melt the permafrost and cause uneven settling, which often 
badly distorts the structures. The tundra has been one of the least exploited biomes, but that era is 
also ending. 

Tropical Rain Forests 

Near the equator, in areas where annual rainfall is greater than about 240 centimeters (95 inches) 
and the mean annual temperature is more than 17° C (64° F), are found the tropical rainforests - 
- the jungles of popular fiction. Tropical rain forests grow in climatic zones where neither 
temperature nor water is a limiting factor. They once covered much of Central America and 
South America, central and western Africa, Madagascar, and Southeast Asia. Tropical rain 
forests are characterized by a great diversity of plant and animal species and by their spacial 

The trees of these forests are taller than those of many temperate forests, the tallest reaching 60 
meters (200 feet) or so. The tops of most trees are intertwined, to form a dense canopy 30 meters 
(100+ feet) or more above the ground ( Figure 4-35A ). The tallest trees stand out above the 
canopy. Beneath it little light penetrates to the ground. There are in many places more than fifty 
species of trees per hectare — many more species than one could find in a day's walk through a 
temperate forest, where the vast majority of trees represent only a few species. Characteristic of 
tropical forests, also, are a wide variety of epiphytes (plants that grow high on the trees in the 
sunlit zone whose roots do not reach down to the soil). Except where a fallen tree has created a 
hole in the canopy to admit light, the understory in a rain forest is sparse, unlike that of a 
temperate forest. Since the trees normally do not branch below the canopy, one can often walk 
almost unobstructed through the dimly lit depths of a mature forest (the forest's edge, in contrast, 
is usually a tangle of riotous growth). Most trees have shallow roots, and many develop huge 
buttresses for support ( Figure 4-35C ). 

Animal species in tropical rain forests are also very diverse, with insects, amphibia, reptiles, and 
birds especially well represented. For example, roughly as many butterfly species can be found 
in a single rain forest locality as are found in the entire United States (five or six hundred), 
including whole groups unknown or scarcely represented in the temperate zones. Coevolutionary 
relationships appear to be more highly developed in this biome than in any other. 

To a layman the luxuriant growth of a tropical jungle implies a rich soil. Nothing, however, 
could be further from the truth. Tropical forest soils are in general exceedingly thin and nutrient- 
poor. They cannot maintain large reserves of minerals needed for plant growth, such as 
phosphorus, potassium, and calcium, primarily 

M. S. Chilarov, Abundance, biomass and vertical distribution of soil animals in different 

zones, in Secondary productivity of terrestrial ecosystem K. Petruzewicz, Warsaw, 1967, p. 

76 T. A. Babb and L. C. Bliss, "Effects of physical disturbance on Arctic vegetation in the Queen 

Elizabeth Islands", Journal of applied ecology, vol. 1 1 ( 1974), pp. 549-562. 

FIGURE 4-35 A tropical rain forest. A. The canopy of a rain forest covering hills in central 

Costa Rica. B. In the dense understory, light penetrates into a rainforest along a stream in 

Sarapiqui, on the Atlantic coastal plain of Costa Rica. C. Buttresses of a rain forest tree in 

Sarapiqui, Costa Rica. (Photos by P. R. Ehrlich.) 


because heavy rainfall and a high rate of water flow through the ground to the water table leach 
them from the soil. The leaching process leaves behind large residues of insoluble iron and 
aluminum oxides in the upper levels of tropical forest soils. 

What nutrients there are in the tropical forest ecosystem are tied up in the lush vegetation itself. 
Those nutrients that are released by the decay of dropped leaves and fallen trees are quickly 
taken up by the shallow network of roots that laces the forest floor and are returned to the living 
vegetation. Since most tropical forest trees are evergreen, the process is continuous. A rich layer 
of humus does not accumulate as it does in a temperate forest, where, after a general leaf- fall, the 
trees are dormant each winter. 

Tropical rain forests are being destroyed more rapidly than any other biome. In Central America, 
the forests are "vaporizing," to borrow the term of Daniel Janzen, one of the most distinguished 


tropical ecologists. In Brazil they are being destroyed as a consequence of direct governmental 

action, in an effort to develop the Amazon basin. Throughout the tropics, forests are being cut 
down to make room for farms. The pressure of human population and ignorance are quickly 
robbing the world of its richest reservoir of terrestrial organic diversity. 

Altitudinal Gradients 

In areas where there are substantial variations in altitude, the terrestrial communities differ at 
different elevations. This is primarily because the temperature of the air decreases about 6° C for 
every 1000-meter increase in altitude, and because, especially in desert areas, rainfall increases 


with altitude. Thus, in the mountains of Colorado above about 3500 meters, temperature 

conditions are similar to those of the tundra biome at sea level, and a treeless alpine tundra 
exists. As one travels northward in the Rocky Mountains, the treeline (the altitude above which 
tundra occurs) gradually lowers until in northern Alaska and Canada it reaches sea level. 

Tundra is virtually continuous in the northern Rocky Mountains; in Colorado "islands" of 
mountain tundra occur as dense archipelagos. Because of this, the faunas and floras of the arctic 
and North American alpine tundras have many similar organisms. For instance, among the 
butterflies of Colorado mountaintops are numerous genera, and even a few species, also found in 
the sea-level tundras 3800 kilometers to the north. By contrast, the treeless zones of tropical 
South American and African mountains, where the treeline is around 4000 meters, superficially 


resemble arctic tundra ( Figure 4-36 ) but show few faunal affinities with the north. For 

example, satyrine butterflies of the genus Erebia found in the high mountains of Colorado are 
representatives of a circumpolar genus characteristic of cool climates. Satyrine butterflies of the 
high Andes that are virtually indistinguishable on the wing from Erebia, however, are members 
of the genus Punapedaloides, which has strictly tropical affinities. 

Thus, in the mountains it is possible to find compressed into 2000 vertical meters a series of 
communities that would occupy a sea-level latitudinal gradient 1000 kilometers or more in 
length ( Figure 4-37 ) . Alpine habitats contribute greatly to the floral and faunal diversity of 
Earth because, although in temperature they closely resemble sea-level habitats closer to the 

poles, they differ in other factors, such as daylength regime and atmospheric pressure. Anyone 
who has been active at an altitude of more than 3500 meters knows that the stresses encountered 
there are quite different from those at sea level anywhere! 

Other Biomes 

In this brief survey, we have not touched on some less extensive but nonetheless interesting 
biomes. Chaparral, found in places with mild marine climates and winter rainfall such as the 
Mediterranean basin and the southwest coasts of North America, South America and 

Janzen, The deflowering. 

For example, see J. Terborgh, Distribution on environmental gradients: Theory and 
preliminary interpretation of distributional patterns in the avifauna of the Cordillera 
Vilcamba, Peru, Ecology, vol. 52 ( 1971), pp. 23-40; and P. H. Whittaker and W. A. Niering, 
Vegetation of the Santa Catalina mountains, Arizona, pt.5: Biomass, production and diversity 
along the elevation gradient, Ecology vol. 56 ( 1975), pp. 771-790. 
S. R. Eyre, Vegetation and soils: A world picture. 


FIGURE 4-36 Above the tree line near La Oroya, Peru, about 4000 meters. (Photo by P. R. 


Australia, and the Cape region of South Africa, is characterized by dry, aromatic, evergreen 
shrubs and shrubby trees ( Figure 4-38 ). Chaparral is subject to periodic fires that maintain the 

dominance of the shrub vegetation; the underground parts of the perennial plants found there are 
fire-resistant, and some of the annuals must undergo fire before germination. In areas such as 
Southern California, people have built homes extensively in chaparral and suffer the 
consequences of the inevitable fires. 

Tropical savannas are grasslands with scattered clumps of trees, which tend to be thorny. This 
biome covers a large portion of Africa and is familiar to all who have seen movies or television 
shows on the magnificent herds of hoofed mammals (and the carnivores that prey on them) that 
still roam in places like the Serengeti Plain. In many regions the savanna is threatened by 
overgrazing by cattle, but some efforts are now being made to develop systems of sustained- 
yield harvesting of native antelopes, which are better adapted to feeding on the savanna plants 
and more resistant to disease than cattle. 

Tropical scrub and tropical deciduous (or seasonal) forest biomes are found where rainfall is 
intermediate between that suitable for savanna and that required for tropical rain forest. The 
scrub (sometimes called thorn forest) is found where the precipitation is between 50 and 125 
centimeters (20 to 50 inches) per year; the deciduous forest, where it is between 125 and 250 
centimeters (50 to 100 inches). The former consists of small hardwood trees, frequently thorny, 
as the name implies. The latter, especially where rainfall is toward the top of the range, is a full- 
scale forest (the monsoon forests of India and Vietnam). In both of these biomes — in contrast to 
tropical rainforests — there are well-defined wet and dry seasons to which both plants and 
animals are adapted. (The reproductive activities of many insects, for example, are timed to 
coincide with the wet season, with its abundance of fresh plant food.) 

A key point to understand here is that an extremely wide variety of situations is covered by the 
term tropical ~ indeed, even by the term tropical forest. The tropics are far more diverse than 
the temperate zones. Consequently, no single system can possibly be devised to solve the 
problems of "tropical agriculture" because those problems vary so enormously from place to 

One further biome, the temperate rain forest, deserves mention. These forests are characteristic 
of the northwestern coast of North America and the southwestern coast of New Zealand, areas 
that receive 200 to 380 centimeters (80 to 150 inches) of precipitation annually. They are 
characterized in North America by very tall coniferous trees such as western hemlock, Douglas 
fir, and redwoods, and have well-developed understories with abundant mosses (there often are 
epiphytic mosses — those that grow on trees and other large plants — as well). This biome is 
extensively exploited for timber, the redwoods being especially valued for their decay-resistant 


FIGURE 4-37 

Altitudinal changes in vegetation in Cement Creek, Gunnison County, Colorado. The sagebrush 

area in the foreground is at about 3000 meters. The second growth of aspens on the slope to the 

left and the coniferous forest on the other slopes are at 3000-3800 meters. Treeless tundra 

occupies the highest areas, at about 4000 meters. (Photo by P. R. Ehrlich.) 

. t&Hfc* 

'•• >rw 

• *' -;: t 


FIGURE 4-38 

Chaparral near El Cajon, east of San Diego, California. Chamise, toyon, Ceanothus, and other 

characteristic chaparral shrubs occupy the hillside; small chaparral oak species dominate the 

denser vegetation along the stream bottom in the foreground. (Photo courtesy of Robert 



Biological communities in fresh water are classically divided into two groups ~ those found in 
the running water of rivers and streams and those of the standing water of ponds and lakes. What 
ties these communities into a logical unit, however, are the physical properties of the medium in 
which they exist. Water, as you will recall from Chapter 2, has extraordinary properties that in 
many ways govern the climate of Earth and the way in which organisms have evolved here. 
Indeed, life as we know it is inconceivable without water. Our discussion of freshwater habitats 
focuses on four properties of water -its change of density at 4° C, its content of dissolved 
oxygen, its transparency, and its content of dissolved chemicals other than oxygen. 

It is fair to say that there would be no freshwater life outside the tropics if H 2 O did not reach its 
highest density above its freezing point. If it did not, ice would sink, and temperate and polar 
bodies of water would freeze solid each winter. In large lakes in the temperate zone, a decrease 
in water density with increasing temperature (above 4° C) creates a layering of water during the 
summer. An upper layer of less dense, warm water, the epilimnion, is formed. Deeper waters 
remain cool and thus more dense, to form the hypolimnion. Separating these two stable layers is 
a zone of rapid temperature change, the thermocline. (For a more detailed discussion, see 
Chapter 11.) This layering and the transparency of the water are intimately related to one of its 
most critical properties, its oxygen content. 

The amount of light that penetrates the water decreases rapidly with depth, the longer (red) 
wavelengths dropping out first. By measuring oxygen production and consumption by plankton 
in water samples, it is possible to determine the point in the gradient of light at which 

photosynthesis is balanced by respiration. This compensation level by convention is 

considered the dividing line between an upper euphotic (autotrophic) zone and a lower profundal 
(heterotrophic) zone. In summer the compensation level is usually above the thermocline; 
because the algae that contribute oxygen as a product of photosynthesis cannot live below that 
point, the amount of dissolved oxygen in the hypolimnion may drop almost to zero. 

The degree of oxygen depletion (summer stagnation) in the hypolimnion is partially a function of 
the productivity of the lake. Lakes with low nutrient content (because of the characteristics of 
their drainage basins, or their ages) tend to have a low density of phytoplankton and few plants in 
their littoral zones (shallow regions where sunlight penetrates to the bottom). In such 
oligotrophia (few foods) lakes, the "rain" of dead organisms and other organic debris into the 
hypolimnion is relatively sparse. Since the degradation of those wastes by decomposers requires 
oxygen, oxygen supplies are less likely to be depleted there than where there is a heavy input of 
wastes. Furthermore, in some young lakes the water is so transparent that the compensation level 
is below the thermocline, and excess oxygen is produced in the hypolimnion by photosynthesis. 
Oligotrophic lakes can thus support populations of such fish as lake trout, which require both 
cool water and considerable oxygen. 

Most oligotrophic lakes are geologically young. As they grow older, a successional process may 
occur as nutrients and sediments wash in and the lakes grow shallower and support more littoral 

vegetation. Productivity increases, and the lakes may have massive blooms of phytoplankton. 
The rain of organic debris into the hypolimnion is heavy, and oxygen depletion there becomes 
severe — especially after the die-off of a plankton bloom. Such eutrophic (well fed) lakes will not 
support some commercially valuable fishes like trout, but will support food fishes like bass and 
bluegills, as well as carp and other "trash fish," all of which are at home in warmer waters and 
tolerant of lower oxygen concentrations. 

Eutrophication is a natural successional process in some lakes, but injections of nutrients in the 
form of pollutants can hasten the process, greatly reducing the value of a lake for recreation and 
fishing (see Chapter 11). Hence, one index of the degree of pollution in a body of fresh water is 
the dissolved oxygen concentration (DO); another is the difference between the production and 
consumption of oxygen — biological (or biochemical) oxygen demand (BOD). The latter may be 
thought of as the amount of oxygen required for the oxidative decomposition of materials in the 
water. The BOD is normally 

The technique is described by E. P. Odum in Fundamentals of ecology, pp. 14-16. 
-160- PAGENUMBER161 

measured in the laboratory as the number of milligrams of O 2, consumed per liter of water over 
a period of five days at 20° C (see Chapter 10 for further discussion of BOD). 

What happens to the stratification of a temperate lake in winter? As the upper layer cools in the 
autumn, it begins to sink, carrying fresh oxygen with it to the depths. The thermocline 
disappears, and nutrient-rich waters from the hypolimnion circulate throughout the lake. At one 
point the lake is isothermal (4° C from top to bottom). Then, when the lake freezes, the 
temperature (and density) gradient is established from the surface ice at 0° C to the rest of the 
lake at 4° C. In spring after the ice melts, the water at the surface warms to 4° C and sinks 
through the lighter 2° to 3° C water below, producing a spring overturn and a redistribution of 
oxygen and nutrients. 

Tropical lakes, in which the water has high surface temperatures, have weak temperature 
gradients and often rather stable layering, created by the resultant slight density differences. 
Therefore, the ecology of those lakes is substantially different from that of temperate lakes. 

The fauna of any body of fresh water, temperate or tropical, will vary somewhat with the 
chemical characteristics of the water. For instance, the pH of the water must be within a certain 
range if certain species offish are to thrive — as every serious fancier of tropical fish has 
discovered. Some species, such as neon tetras, come from acid waters and do best in an aquarium 
if the pH is kept low. Others, such as mollies, do best in more alkaline water (high pH). Aquatic 
plants are similarly sensitive to pH. The concentration of calcium ions has been shown to be 
important in restricting the distribution of freshwater sponges and to be related to the distribution 
of triclad flatworms (planarians), but much still remains to be learned about the relationship 


between the chemistry of water and its biota. This is becoming especially important as human 

beings increasingly change the chemistry of fresh waters with pollutants. 

Streams and rivers differ from ponds and lakes primarily in their relative lack of stratification in 
either temperature or oxygen concentration, in their having currents, and in their proportionally 
greater interface with the land (the length of banks relative to the volume of water). These 
differences strongly influence the biota. Usually in streams producers are inadequate to supply 
the consumers, the difference being made up by organic materials washed from the banks. 
Because of the currents, most of the plants and animals of streams and rivers have evolved ways 
to keep from being washed downstream ~ using holdfasts, burrowing into the substrate, or 
having superb swimming ability (like brook trout). Plankton is comparatively rare in running 
water, but in the slower-moving parts of streams and rivers it may be an important part of a 

The normally high oxygen content of streams and rivers has meant that animals adapted to 
running water generally have little tolerance for low oxygen concentration. Any addition of 
oxygen-reducing (high BOD) pollutants to streams or rivers is therefore likely to have a drastic 
effect on the fauna. 


The oceans cover about 71 percent of Earth's surface, thus making them the most extensive 
habitat. As we described in Chapter 2, the oceans play major roles in the hydrologic cycle and in 
the global weather machine; therefore, their influence on life extends to all organisms. 
Considering their size, the oceans are an extraordinarily uniform environment — especially the 98 
percent or so of their volume that is below the depth to which enough light penetrates for 
photosynthesis to occur. 

Because the oceans are rather thoroughly stirred by currents, there is sufficient oxygen for 
heterotrophs in most areas, as well as abundant CO 2 for autotrophs. But nutrients are often in 
short supply, especially nitrogen and phosphorus. Productivity is highest in the oceans in areas of 
upwelling, as along the western coast of South America, where nutrients are brought to the 
surface in abundance. In those areas phytoplankton reproduce without the nutrient shortage that 
restricts much of the open ocean to low productivity. 

Many bizarre and wonderful creatures have evolved in the sunless depths of the oceans, feeding 
on the organic matter that drifts down from above and often signaling one another with 
luminescent organs. Most of the 

J1 Krebs, Ecology, pp. 98-101. 


n Ti* tor*l MATER COLUMN 

muonuis of cahsg* fusd 






»«1 (SO 



<» ?w 


54 IH 

no AM 


9» 237 



r*. Ma -l tttai 



FIGURE 4-39 Gross primary production per unit area in the Pacific Ocean. Net primary 

production would be about 60 percent of the figures shown. (Data from NAS/NRC, Resources 

and man) 
■162- PAGENUMBER163 

biological activity of interest to humanity in the oceans, however, occurs near the surface in the 
euphotic zone (where photosynthesis exceeds plant respiration) and especially near the 
continents, where the availability of nutrients maximizes productivity. That is where the great 
fisheries are, and that is also where human inputs to the oceans are greatest ( Figure 4-39 ). 

Near the shore the marine environment becomes more complex, and intricate zonations have 
developed under the influence of light gradients, wave action, and tides ( Figure 4-40 ). The most 
complex, and perhaps the most interesting, marine habitats are the coral reefs that fringe the 
shores of tropical landmasses. The reefs are made of the skeletons of coral animals that live in a 
mutualistic relationship with algae. In some ways the complexity of the reef habitat rivals that of 


a tropical rain forest, with vertical stratification and great diversity of species. Roughly one- 
third of all fish species (both freshwater and saltwater) live on coral reefs ( Figure 4-41 ). 
















(Snails resistant to dessication) PYURA STOLONIFERA 

(Ascidian sea squirt) Short algal turf 


FIGURE 4-40 Intertidal zonation on the south coast of Africa. A. The upper edge of the Littorina 

snail zone. B. The upper edge of the barnacle zone. C. The upper edge of the zone defined by the 

limpet Patella cochlear. D. The upper edge of the subtidal zone. The shore is shown at an 

exceptionally low spring tide on an unusually calm day. The zones are represented in a very 

diagrammatic fashion, somewhat telescoped; the Littorina zone is especially reduced. (Adapted 

from T. A. Stephenson and A. Stephenson, Life between tidemarks on rocky shores, W. H. 

Freeman and Company. Copyright © 1972.) 


Paul R. Ehrlich, "Population biology of coral reef fishes", Annual Review of Ecology and 
Systematics, September 1975, pp. 211-247. 



*jg$, . . 



FIGURE 4-41 

Complexity and diversity of a coral reef habitat is indicated by this diagrammatic cross-section 
of reefs off St. John, U.S. Virgin Islands, showing general distribution of 73 fish species, day and 
night. Reefs in the northern part of the Australian Great Barrier Reef can support up to 1000 fish 

species. On coral 

J -^ J ^PSfe. J tfej-L 

T* E» 

SA5J fe£Ststojt=rf^ 

reefs, of course, them are a great many kinds of invertebrates and algae not indicated here. 
(Adapted from "Results of the tektite program: ecology of coral reef fishes", Natural History 
Museum, Los Angeles County, Science bulletin, 14, 1972.) 

But even the diverse coral reef fauna shows great similarities in such far-apart places as the 
Society Islands of the central Pacific and the reefs fringing the East African coast — and genera 
of fishes found in both of those places also occur in the Caribbean. In contrast to both terrestrial 
and freshwater habitats, which are often isolated from each other, the oceans have a great unity. 


Where rivers and streams flow into the oceans, and fresh and salt water intermingle in tidal 
ponds, rivers, and embayments are found the habitats known as estuaries. Organisms that live in 
estuaries must be tolerant of a wide range of salinities and usually of a wide range of 
temperatures also. The daily rhythm of the tides gives them an ever-changing environment. 

Many organisms have evolved to live in these conditions, and the productivity of estuaries 
(which tend to be nutrient traps) is very high ( Table 4-6 ). Some of this productivity is harvested 
at the estuary by human beings — most of the oysters and crabs people eat, for example, are 
estuarine. More important to Homo sapiens, however, is the role estuaries play as "nurseries" for 
commercially important marine species. Many of the shrimps and fishes that are harvested at sea 
have estuarine young. Indeed, it has been estimated that about twothirds of the rich commercial 
fishing on the continental shelf of the eastern United States consists of species that spend part of 
their life cycles in estuaries. — 

Another estimate credits each hectare of estuary with contributing more than 550 kilograms of 

fishes per year to coastal fisheries. Using a conservative estimate of 1.4 X 10 6kM 2 (1.4 X 108 
hectares) of estuaries worldwide, their total annual contribution to fisheries would be 550 X 1.4 
X 10 g = 77 X 109 kg = 77 million MT. That is somewhat more than the present global fisheries 
harvest and about one-third of the total estimated potential production of fishes in the sea (see 
Chapter 7). That estuaries are among the most threatened of all habitats today is therefore cause 
for considerable concern. 


One further habitat deserves attention — one that contains only a small amount of land that isn't 
perpetually ice-covered and that supports a "terrestrial" community almost entirely dependent on 
marine producers. That habitat is Antarctica. There are two flowering plants (a grass and a plant 
of the carnation family) and a variety of mosses and lichens scattered around the fringes of the 
Antarctic continent. These producers support small communities of invertebrate consumers and 
decomposers, but all of the vertebrates found on land in the Antarctic feed on food chains based 
on massive summer blooms of marine phytoplankton, supported by extremely nutrient -rich 
upwellings in the sea. The phytoplankton support large populations of shrimplike krill, which in 
turn form the feeding base for animals as diverse as baleen whales and penguins ( Figure 4-42 ). 

Penguins and other seabirds breed on land in the summer in enormous colonies that in their 

turn support populations of skuas (predatory relatives of seagulls), which feed on penguin eggs 
and young penguins. One antarctic seal, the leopard seal, feeds on penguins. Another, the 
Weddell seal, feeds on fishes and squid, and a third, the crabeater seal, belies its name by feeding 
mostly on krill. In turn, those seals are preyed upon by killer whales ( Figure 4-43 ). 

Of all the major habitats on Earth, Antarctica, in a physical sense, is the least disturbed by human 
activities. Antarctica is protected by a barrier of sea ice, so only a handful of people had ever set 
foot on the continent before the middle of the twentieth century. And, except for the small huts 

of explorers like Scott and Shackleton, the bits of land not permanently ice-covered showed no 
signs of human disturbance before that time. 

J. L. McHugh, Management of estuarine fisheries, in A symposium on estuarine fisheries, 
American Fisheries Society, Washington, D.C., 1966, special publication 3; G. M. Woodwell, 
P. H. Rich, and G. A. S. Hall, Carbon in estuaries, in Carbon and the biosphere, Woodwell 
and Pecan, eds., p. 234. 

R. H. Stroud, in Symposium on the biological significance of estuaries, P. A. Douglas and R. 
H. Stroud, eds., Sport Fishing Institute, Washington, D.C., 1971, p. 4. 
The exception to this is the emperor penguin, which breeds in winter on the ice. 



FIGURE 4-42 
Antarctic food chains. Notice that even the "terrestrial" vertebrates are dependent upon marine 


But long before World War II, humanity was exploiting the antarctic oceanic ecosystem that is 
the feeding base of all large animal life on that continent. Early in the twentieth century, whaling 
in antarctic waters was so intense that by 1920 the right whale, so-called because it was easy to 
catch and floated after harpooning (the "right" whale from a whaler's point of view!) was 
virtually extinct, and concern was expressed in the international community about the future of 
all exploited whales. After World War II, overexploitation continued (see Chapter 7 for more 
detail); today whales are scarce in antarctic waters. In a two-week voyage in antarctic waters in 
1974/ 1975, the authors saw no whales of the species that have constituted the backbone of the 
whaling industry. In the very same waters in 1912, Robert Cushman Murphy reported seeing 
"whales in all directions," even though the slaughter of those great mammals was far enough 
advanced by then that the shoreline near the whaling station at South Georgia Island was "lined 


for miles with the bones of whales." — 

What effect the removal of the baleen whales from the Antarctic Ocean will have on the structure 
of the 

Logbook for grace, Time-Life Books, New York, 1965, p. 188. 
■167- PAGENUMBER168 

C FIGURE 4-43 Antarctic food chains. A. Crabeater seals on an ice floe in Paradise Bay, 

Antarctica. In the absence of large terrestrial predators, these seals, unlike those of the Arctic 

(which are eaten by polar bears and hunted by Eskimos), have not evolved a fear of being 

approached. B. Trilobed teeth of a crabeater are used in capturing krill. C. A Weddell seal under 

attack by a pod of killer whales in Lemaire Channel, Antarctica. The seal can be seen on the ice 

just behind the head of the leading whale, which has risen from the water to look at it. Repeated 

coordinated rushes of the whales, which sent streams of water over the ice floe, eventually 
washed the seal into the sea, where it was devoured. (Photos by P. R. Ehrlich.) 

FIGURE 4-44 
A garbage dump at U.S. Palmer Station in Antarctica. (Photo by P. R. Ehrlich.) 
■168- PAGENUMBER169 

B FIGURE 4-45 A. A small portion of an Adelie penguin colony in Hope Bay, Antarctica. Like 

many human societies (such as the Netherlands), breeding penguin colonies are utterly 

dependent upon imported resources for their survival. B. An Adelie parent, having returned from 

the sea, regurgitates krill for its chick. Notice the spined tongue, which helps the bird to grasp the 

krill, and compare with Figure 4-43 B . (Photos by P. R. Ehrlich.) 

ecosystem there remains to be investigated, but it must be considerable. At one time an estimated 
100,000 individuals of the now virtually extinct blue whale roamed antarctic waters for four 
months of every year. Each individual consumed four tons of plankton, mostly krill (Euphausia 
superba), per day; hence, the blue whale alone processed some 50 million tons of plankton 
annually. The dramatic reduction in the number of whales has in theory made an enormous 
surplus of plankton available to the seals, penguins, and fishes that also feed on it, but no studies 
have been made of possible changes in their population sizes. Hence, humanity, through whaling 
alone, has clearly had a dramatic impact on the biology of Antarctica. 

In the years following World War II, land stations have been established on the antarctic 
continent by various nations. Some were attempts to claim sovereignty; many in the mid-1950s 
were cooperating in scientific research projects of the United Nations International Geophysical 
Year. These stations have produced local versions of the kind of environmental deterioration so 
commonplace in the rest of the world. The United States has produced an ugly town at McMurdo 
Sound, symbolically complete with a nuclear power plant, and at the Palmer Station on the 
Antarctic Peninsula, a typical American garbage dump ( Figure 4-44 ), the largest on the 
continent. More serious than such local blights is the exploration for minerals that someday may 
open the Antarctic to the kind of destruction now being visited on the Arctic. Recent U.S. Navy 
reports of 45 billion barrels of oil in Antarctica hardly is a hopeful sign. — 

Another hazard to the antarctic environment is the possible escape of dogs from stations. 
Antarctica is devoid of large terrestrial predators, and penguins in particular could easily be 
decimated. We have seen dogs killing them at the Argentine station at Hope Bay. The penguins 
nest in vast rookeries on the ground ( Figure 44-5 ) ; they would be sadly vulnerable to even one 
or two feral dogs, which could do enormous damage before perishing in the antarctic winter. 

Finally, of course, antarctic ecosystems have been polluted with chlorinated hydrocarbons, lead, 
and other 

%1 "Today" Show, NBC, February 25, 1975. 

products of civilization. Thus, although to the untrained eye the continent still seems remote and 
untouched, its biology has already been profoundly modified by human action, and further 
destructive change is threatened. 


Because ecology often deals with exceedingly complex systems, it is a standard technique in the 
science to attempt to gain understanding by creating models of them. A model can be thought of 
as a simplified representation of reality. Ideally, it should be an aid to thinking about reality and 
should lead to an ability to make predictions about future states of reality and suggest 
experiments or measurements that could help define reality more precisely. For example, the 
logistic curve is a model of the growth processes of real populations in certain circumstances. 
You will recall that in our earlier discussion it was simplified in several ways — for instance, it 
assumed that all individuals were ecologically identical, which is never true. Models can be 
verbal or graphic (or hydraulic or electric), but many of the most useful are mathematical. 

Mathematical models fall into two general classes -they either include the random changes that 
seem to characterize nature or they ignore them. The former are statistical or stochastic models; 
the latter, deterministic. The logistic curve is a deterministic model. 

In constructing a model, one can strive to build into it one or more of the following properties: 
precision, realism, generality, and manageability. Precision is the ability of a model to predict 
future states of the system accurately. Realism is the extent to which the mathematical 
formulation of the model reflects the underlying biological processes of the system modeled. 
Generality is the degree to which the model applies to diverse systems. Manageability is the ease 
with which the model can be manipulated ~ for example, are the differential equations easy to 
solve, or are their solutions easy to approximate? The logistic curve model is high in 
manageability and generality, medium in realism, and low in precision. 

Two general schools of model-building have developed in ecology: one dealing with complex 
computer models (systems ecology) and one dealing with simple analytic models. Systems 
ecology has concentrated on constructing computer models of ecosystems. The system is broken 
down into a series of compartments, such as herbivores, top carnivores, cities, and farms; and 
numbers (system variables) are used to describe the state of each compartment at a given time 
(say, the biomass present). Functions (transfer functions) are then produced to describe the 
interactions of the compartments, such as the flows of energy among them. Inputs from outside 
of the system that are not influenced by the system, such as arriving solar energy or fossil fuels, 
are described by other functions (forcing functions). 

The model is then manipulated in attempts to make it simulate the past behavior of known 
systems satisfactorily and to see what elements of the system are especially sensitive or 
insensitive to changes elsewhere in the system. One simple systems model of the general type is 
shown in Figure 4-46 , in which the system is analogized to an electric circuit. The most famous 
model of this class was the world model developed by Donella and Dennis Meadows and their 
colleagues at Massachusetts Institute of Technology and published in The Limits to Growth. — 
That study is discussed in greater detail in Chapter 12. 

The computer models of the systems ecologists tend to be high in precision and low in generality 
and to lose reality as they become more general. Whereas they usually do not supply rewarding 
holistic insights, they can often provide clues as to how and where an ecological system might 
most profitably be manipulated and the converse, where a system is so robust that manipulation 

is likely to have little effect. One of the major drawbacks of systems ecology is the relative 
paucity of data available with which to develop and test models. 

The modeler of the analytic school has a different approach. He or she would start with a 
question such as, "What would be the form of a population growth curve if the rate of increase 
were a linear function of size itself?" Or the question might be, "Under what circumstance of 
resource availability would an organism that can use two 

Donella H. Meadows; D. L. Meadows; J. Randers; W. W. Behrens, III , The limits to growth, 
Universe Books, Washington, D.C., 1972. 













FIGURE 4-46 

A simple agricultural system in Uganda. Food is derived from grains, meat, blood, and milk 

Animals serve as storage units. The numbers represent kilocalories per square meter per year. 

Dry weights were converted to kilocalories, using 4.5 kcal per gram. There are 70 people per 

square mile at 150 pounds per person, 25 percent of which is dry matter, which is equivalent to 1 

gram per square meter. 

The basic net production of plant material for dry regions is a function of rainfall. Using 21 
inches of rainfall, one is able to determine that 500 grams of dry plant material are produced per 
square meter of land each year. Since I acre is cultivated per person and there are 70 persons per 
square mile, 1 1 percent of the natural yield area is preempted by crops. At 4.5 cattle per person 
and 560 pounds per cow, 33 percent of which is dry weight, excluding ash and water, 10.2 grams 
of animal weight is produced per square meter. 

By considering the monthly consumption of milk, blood, and meat, one obtains an annual caloric 
yield per person from the cattle of 3800 kcal of milk, 2450 kcal of meat, and 1265 kcal of blood, 
which provides the per-area data in the figure. The caloric requirement per person is given as 
2000 kcal per person per day, or 19.7 kcal per square meter per year. The milk, meat, and blood 
supply only 0.2 of this requirement, so crops must provide the remaining 19.5, a net yield much 
less than the net yield of vegetation of the natural range. Total insolation in this area just above 
the equator is about 4000 kcal per square meter per day. 

The work of men in tending the crops and cattle can be taken as a percentage of their time spent 
in this activity (primarily during the daylight hours). As the culture is intimately involved with 
the cattle, it is assumed that one-sixth of the daily metabolism of each man is devoted to 
management of the cattle and an equal amount is used for production of crops. The rationale is 
that the maintenance requirements of a man during his work are necessary to that work, in 
addition to calories directly expended. 

The metabolic activites of a 650-pound steer requires 8000 kcal per day, or 365 kcal per square 
meter per year. Some fraction of the steer's time and metabolism goes into refertilizing the range 
on which it grazes, thus reinforcing and maintaining its nutrient loop. Part of a steer's day is 
spent on the move, and parts of its organ systems are involved in the nutrient regeneration 
system. One-tenth of its metabolism was taken as its work contribution to vegetation stimulation. 

This system does not involve money, and an economic transactor symbol does not appear. 
(Slightly modified from President's Science Advisory Committee Report on the World Food 
Problem, 1967.) 


classes of resources win out in competition with one that can use three?" Mathematical models of 
the situation (like the logistic curve in the first instance) are constructed, and then the model and 
its assumptions are tested against actual systems in nature or in the laboratory. 

The models of this school tend to be high in generality and low in precision, although in some 
simple cases the reverse can be true. Realism, which is generally lower than in systems ecology, 
also tends to decline with increasing generality. 

Although both schools in a sense are in their infancies, it is fair to say that systems ecology is 
likely to be most productive in providing tools for the solution of practical problems, whereas 
simple analytic models probably will be most valuable in helping ecologists understand the more 
general properties of ecological systems. Both schools have provided a welcome infusion of 
theory into academic ecology. What is badly needed now is a greater concern for applied 
problems among academic ecologists and a greater appreciation of the potential value of theory 
among agriculturalists, wildlife managers, city planners, politicians, and others directly 
concerned with ecological problems. 

Recommended for Further Reading 

Collier, B. D.; G. W. Cox; A. W. Johnson; P. C. Miller. 1973. Dynamic ecology. Prentice-Hall, 
Englewood Cliffs, N.J. A fine modern text with a systems approach and better-than-average 
integration of plant ecology with the other material. 

Ehrlich, Paul R.; R. W. Holm; and D. R. Parnell. 1974. The process of evolution. 2d ed. 
McGraw-Hill, New York. A basic textbook of evolutionary theory. 

Emlen, J. M. 1973. Ecology: An evolutionary approach. Addison-Wesley, Reading, Mass. A 
modern text especially suitable for those with a mathematical bent. (See especially the section on 
Leslie matrix analysis of population dynamics.) 

Krebs, C. J. 1972. Ecology: The experimental analysis of distribution and abundance. Harper, 
New York. A text in the tradition of Andrewartha and Birch (see Additional References). Well 
written and especially good on population dynamics. 

Odum, E. P. 1971. Fundamentals of ecology. 3d ed. W. B. Saunders, Philadelphia. The most 
recent edition of a classic ecology text. Comprehensive, with an excellent bibliography. 

Pianka, E. R. 1974. Evolutionary ecology. Harper, New York. A relatively brief, well-integrated 
text, taking the analytical-theoretical approach. 

Pielou, E. C. 1974. Population and community ecology: Principles and methods. Gordon and 
Breach, New York. Excellent with a detailed mathematical approach. 

Ricklefs, R. E. 1973. Ecology. Chicago Press, Newton, Mass. A comprehensive text outstanding 
for its integration of population genetics with ecology. 

Roughgarden, J. In press. Theory of population genetics and evolutionary ecology, an 
introduction. Macmillan, New York. The best brief text on the mathematical theory of evolution 
and ecology. Highly recommended. 

Tait, R. V., and R. S. De Santo. 1972. Elements of marine ecology. Springer-Verlag, New York. 
A sound introduction. 


Watt, K. E. F. 1968. Ecology and resource management. McGraw-Hill, New York. This is a 
good source for basic material on the analysis of exploited populations and the systems approach 
to theory. 

Whittaker, R. H. 1975. Communities and ecosystems . 2d ed. Macmillan, New York. An excellent 
brief text with well-balanced, good coverage of ecological problems. 

Additional References 

We list here some of the works written since World War II which provide access to the vast 
literature of ecology and evolutionary biology. In addition to these there are many journals in 
which articles pertinent to subjects covered in this chapter appear. Key journals include: 
American naturalist (U.S.), Biotropica (U.S.), Ecology (U.S.), Ecological monographs (U.S.), 
Ekologiya (USSR, available in translation as Soviet journal of ecology), Environmental 
entomology (U.S.), Evolution (U.S.), Genetica (Netherlands), Genetics (U.S.), Heredity (UK), 
Journal of Ecology (UK), Journal of experimental marine biology and ecology (Netherlands), 
Journal of theoretical population biology (U.S.), Marine biology (West German Federal 
Republic), Nature (UK), Oecologia (West German Federal Republic), Oikos (Denmark), 
Researches on population ecology (Japan), and Science (U.S.). 

Allee, W. C; A. E. Emerson; O. Park; T. Park; and K. P. Schmidt. 1949. Principles of animal 
ecology. Saunders, Philadelphia. This massive work, known to generations of students as the 
Great Aepps, is far out of date but has a fine bibliography of the older literature. 

Andrewartha, H. G., and L. C. Birch. 1954. The distribution and abundance of animals. 
University of Chicago Press, Chicago. The modern era of animal population ecology begins with 
this classic. A "must" for all those professionally interested in the environment. 

Annual Reviews, Inc., Annual review of ecology and systematics. Published yearly since 1970. 
Annual Reviews, Inc. Palo Alto, Calif. All volumes contain articles of great interest to ecologists 
and evolutionists. 

Baker, H. G., and G. L. Stebbins, eds. 1965. Genetics of colonizing species. Academic Press, 
New York. A basic source with many first-rate contributions. 

Bartlett, M. S., and R. W. Hiorns, eds. 1973. The mathematical theory of the dynamics of 
biological populations . Academic Press, London. Only for the mathematically sophisticated. 

Boer, P. J. den; and G. R. Gradwell, eds. 1971. Dynamics of populations . Center for Agricultural 
Publishing and Documentation, Wageningen, the Netherlands. A fine sample of recent work on 
diverse topics. 

Cavalli-Sforza, L. L., and W. F. Bodmer. 1971. The genetics of human populations . W. H. 
Freeman and Company, San Francisco. A fine, comprehensive source on human and population 

Chambers, K. L., ed. 1970. Biochemical coevolution. Oregon State University Press, Corvallis. 
Papers on important aspects of coevolution. 

Cloudsley- Thompson, J. L. 1975. Terrestrial environments. Wiley, New York. Up-to-date 
descriptions of biomes, zoogeography, and so forth, including the freshwater biome. 

Creed, R., ed., 1971 . Ecological genetics and evolution. Blackwell, Oxford. A fine volume 
focusing on the ecological causes of genetic polymorphism. 

Crutchfield, J. A., and G. Pontecorvo. 1969. The Pacific salmon fisheries: A study of irrational 
conservation. Johns Hopkins, Baltimore. Deals with interactions of biological and economic 
yields in exploited populations. 

Dobben, W. H. van, and R. H. Lowe-McConnell, eds. 1975. Unifying concepts in ecology. Dr. 
W. Junk, The Hague. Papers from the First International Congress of Ecology on energy flow, 
productivity, diversity and stability, and ecosystem management. 

Dobzhansky, T. 1970. Genetics of the evolutionary process. Columbia University Press, New 
York. An important source for both theoretical and experimental approaches to population 
genetics ~ a successor to Dobzhansky's classic, Genetics and the origin of species ( New York; 
Columbia University Press, 3d edition, 1951). A must for anyone interested in evolution. 

Elton, C. S. 1958. The ecology of invasions by animals and plants. Methuen, London. The source 
on this subject — a classic. 

1966. The pattern of animal communities. Methuen, London. Natural history, with much 

fascinating material by one of the greatest of all ecologists. 

Emden, H. F. van, ed. 1973. Insect-plant relationships. Blackwell, Oxford. The bibliographies of 
many of these interesting papers are extensive and useful. 

Etherington, J. R. 1975. The environment and plant ecology. Wiley, New York. A good basic 
text in physiological ecology of plants. 

Eyre, S. R. 1963. Vegetation and soils: a world picture. Aldine, Chicago. This comprehensive 
work includes a section on the impact of human beings. 

-173- PAGENUMBER174 

Ford, E. B. 1964. Ecological genetics . Methuen, London. The classic of the British school 
working with natural populations. 

Gilbert, L. E., and P. H. Raven, eds. 1975. Coevolution of animals and plants. University of 
Texas, Austin. An excellent collection dealing with a key field in population biology. 

Heywood, V. H., ed. 1973. Taxonomy and ecology. Academic Press, London. A wide-ranging 
symposium with several important papers on coevolution. 

Hutchinson, G. E. 1958. Concluding remarks. Cold Spring Harbor symposium on quantitative 
biology vol. 22, pp. 415-427. The Biological Laboratory, Cold Spring Harbor, New York. A key 
paper which forms the basis of much recent work on niche theory. 

1969. Eutrophication, past and present. In Eutrophication: Causes, consequences, 

correctives, National Academy of Sciences, Washington, D.C. A fine article by one of the world's 
most distinguished ecologists and limnologists. 

Kendeigh, S. C. 1974. Ecology. Prentice-Hall, Englewood Cliffs, N. J. See for material on 

Kershaw, K. A. 1973. Quantitative and dynamic plant ecology. 2d ed. American Elsevier, New 
York. Excellent on statistical aspects, but like many other treatments of plant ecology totally 
ignores the critical role of animals in plant ecology. 

Kozlowski, T. T., and C. E. Allgren. 1974. Fire and ecosystems. Academic Press, New York. 
Covers the ecological roles of fire, both harmful and beneficial. 

Kummel, B. 1970. History of the earth. 2d ed. W. H. Freeman and Company, San Francisco. A 
comprehensive introduction to historical geology — a good source for those interested in the 
course (rather than the process) of evolution. 

Lewontin, R. C, ed. 1968. Population biology and evolution. Syracuse University Press, New 
York. A good collection. 

1974. The genetic basis of evolutionary change. Columbia University Press, New York. 

Exciting summary and synthesis of recent work on enzyme polymorphisms and their significance 
in evolutionary theory. 

Lieth, H. 1975. Primary productivity in ecosystems: Comparative analysis of global patterns. In 
Unifying concepts in ecology, W. H. van Dobben and R. H. Lowe-McConnell, eds., pp. 67-88. 
Estimates about 122 x 199 metric tons of terrestrial primary productivity (55 x 109 tons of 

MacArthur, R. H. 1972. Geographical ecology: Patterns in the distribution of species. Harper, 
New York. A pioneer synthesis dealing with the factors controlling the distribution of organisms 
~ the last work of a great pioneer in the "new ecology," published shortly before his untimely 

McNaughton, S. J., and L. L. Wolf. 1973. General ecology. Holt, Rinehart and Winston, New 
York. An excellent text interweaving theory and examples from a very wide range of organisms. 

Margalef, R. 1968. Perspectives in ecological theory. University of Chicago Press, Chicago. 
Offbeat and interesting. 

May, R. M. 1973. Stability and complexity in model ecosystems. Princeton University Press, N.J. 
A theoretical consideration of the crucial problem of the resistance of ecosystems to perturbation. 

Mayr, E. 1963. Animal species and evolution. Harvard University Press, Cambridge, Mass. This 
scholarly and exhaustive treatise supercedes the author's earlier classic, Systematics and the 
origin of species ( Columbia University Press, 1942). Animal species and evolution, has been 
abridged as Populations, species and evolution ( Harvard University Press, 1970). Highly 

Murdoch, William W. 1975. "Diversity, complexity, stability and pest control". Journal of 
applied ecology, vol. 12, pp. 795-807. A key article disputing the classic view that diversity 
promotes stability. 

National Academy of Sciences (NAS). 1969. Eutrophication: Causes, consequences, correctives. 
NAS, Washington, D. C. A gold mine of information ~ see especially G. E. Hutchinson's 
introductory article. 

Oosting, H. J. 1956. The study of plant communities. 2d ed. W. H. Freeman and Company, San 
Francisco. A classic, much of it now out of date. 

Orians, Gordon H. 1975. Diversity, stability and maturity in natural ecosystems. In Unifying 
concepts in ecology, W. H. van Dobben and R. H. Lowe-McConnell, eds. A good, brief 
discussion suggesting that "attempts to find general relationships between diversity and 'stability' 
are likely to be fruitless." 

Patten, B. C, ed. 1971. Systems analysis and simulation in ecology. Academic Press, New York. 
A two- volume collection. 

Pielou, E. C. 1969. An introduction to mathematical ecology, Wiley, New York. A fine treatment 
for those quantitatively inclined. 

Pimentel, D. 1973. "Extent of pesticide use, food supplies, and pollution". Journal of the New 
York Entomological Society, vol. 81, pp. 13-33. Reports increases in crop losses despite 
increased pesticide use, due in part to the "practice of substituting insecticides for sound 
bioenvironmental pest control" (for example, crop rotation and sanitation) and also to higher 
consumer standards. 

Pomeroy, L. R., ed. 1974. Cyles of essential elements. Dowden, Hutchinson and Rose, 
Stroudsburg, Pa. Reprints of key papers. 

Poole, R. W. 1974. An introduction to quantitative ecology. A good introduction to mathematical 
analyses in ecology. 

Raven, P. H.; R. F. Evert; and H. Curtis. 1976. Biology of plants. Worth, New York. By far the 
best modern botany text. 

Reichle, D. E., ed. 1970. Analysis of temperate forest ecosystems. SpringerVerlag, New York. A 
fine recent summary, well integrated for a collection of separate papers. 

J. F. Franklin, and D. W. Goodall, eds. 1975. Productivity of world ecosystems. National 

Academy of Sciences, Washington, D.C. Report of a symposium with many important papers. 

Richards, P. W. 1952. The tropical rainforest. Cambridge University Press. A standard work, 
still very useful. 

1973. The tropical rain forest. Scientific American, December. A good overview of the 

ecosystem and its exploitation. 

Rodin, L. E., and N. I. Brazilevich. 1967. Production and mineral cycling in terrestrial 
vegetation. Oliver and Boyd, London. This comprehensive review was first published in the 
USSR in 1965. 

and N. N. Rozov. 1975. Productivity of the world's main ecosystems. In Productivity of 

world ecosystem, D. Reichle, J. Franklin , and D. Goodall. National Academy of Sciences, 
Washington, D.C., pp. 13-26. Higher estimates than R. H. Whittaker and G. E. Likens. 

Simberloff, D. S., and L. G. Abele. 1975. Island biogeography theory and conservation practice. 
Science, vol. 191, pp. 285-286 (January 23). In some cases a cluster of small refuges may 
support more species than one large one. 

Simpson, G. G. 1953. The major features of evolution. The most thorough and general 
explanation of evolutionary trends in animals, but now out of date. 

Slobodkin, L. B. 1961. Growth and regulation of animal populations . Holt, Rinehart and 
Winston, New York. Partly out of date, but a landmark in its day. 

-174- PAGENUMBER175 

Sondheimer, E., and J. B. Simeone, eds. 1970. Chemical ecology. Academic Press, New York. 
One of the first volumes in a rapidly expanding field. 

Southwood, T. R. E. 1966. Ecological methods. Methuen, London. The source on techniques for 
work on populations of small animals. 

Spurr, S. H., and B. V. Barnes. 1973. Forest ecology. 2d ed. Ronald, New York. A first-rate text. 

Stebbins, G. L. 1950. Variation and evolution in plants. Columbia University Press, New York. 
Old but still essential; some more recent material may be found in his Chromosomal evolution in 
higher plants, Addison-Wesley, Reading, Mass., 1971. 

1974. Flowering plants: Evolution above the species level. Harvard University Press, 

Cambridge, Mass. A recent tour de force on the relationships of groups of plants. Essential for 
those interested in plant-animal coevolution, a topic of great importance to humanity. 

Usher, M. B., and M. H. Williamson, eds. 1974. Ecological stability. Halstead Press, New York. 
A collection illustrating the diversity of views ecologists hold on this topic. 

Van G. M. Dyne 1969. The ecosystem concept in natural resource management. Academic 
Press, New York. A useful compendium with numerous examples of the analysis and modeling 
of ecosystem. 

Watson, A., ed. 1970. Animal populations in relation to their food resources. Blackwell, Oxford. 
A symposium on the feedback from food resources to the regulation of numbers. 

Weller, J. M. 1969. The course of evolution. McGraw-Hill, New York. A survey of the 
evolutionary history of major plant and animal groups. 

Whittaker, R. H., and G. E. Likens. 1973. Carbon in the biota. In Carbon and the biosphere, G. 
Woodwell and E. Pecan, eds. Technical Information Center, USAEC, Washington, D.C., pp. 
281-300. Discussion of productivity, previous estimates, and impact of man. 

(See also Rodin, Bazilevich, and Rozov, Productivity of the world's main ecosystems?) 

Wilson, E. O. 1975. Sociobiology. Harvard University Press, Cambridge, Mass. This landmark 
volume has much to say about the ecology and evolution of social animals, from social insects to 
human beings. Extremely controversial, with a superb bibliography. 

Woodwell, G. M., and H. H. Smith, eds. 1969. Diversity and stability in ecological systems. 
USAEC, Washington, D.C., Brookhaven Symposia in Biology, 22. A collection of papers that 
bears witness to the complexity of the problem and the diversity of approaches to it. 

Woodwell, G. M. and E. V. Pecan, eds. 1973. Carbon and the biosphere. USAEC, Oak Ridge, 
Tenn. Numerous up-to-date articles pertinent to this chapter and chapters 2 and 3. 


[This page intentionally left blank.] 



Population and Renewable Resources 


The tide of the earth's population is rising, the reservoir of the earth's living resources is falling. 

- Fairfield Osborn, 1948 

Having considered the basic principles of operation of the environment in which human society 
is embedded, we turn now to the human population itself and certain of the demands it makes on 
that environment. 

Chapter 5 gives a rather detailed introduction to demography — the study of the size, structure, 
and distribution of the human population and the ways and reasons these characteristics change. 
Some knowledge of the history of human population change is essential to an understanding of 
the present situation, and this is provided. We attempt, as well, to furnish some basis for 

estimating what future patterns of population change may be, while recognizing that all forecasts 
should be heavily qualified and surrounded by disclaimers. (Demographers in the past have been 
notably more successful at describing what has happened than at predicting what will happen.) 
Of course, there are some kinds of changes that, barring catastrophe, are extraordinarily unlikely 
~ for example, a leveling-off of the global population size in less than several decades and at a 
size much smaller than about twice today's. We explore in detail the reasons for this unfortunate 
human "commitment" to further growth. 

The primary reasons for concern with population are the pressures that population characteristics 
and population change impose on physical resources, environmental services, economic 
prosperity, social systems, and human values. The natures of many of those pressures are 
examined in subsequent chapters, but it is worth setting down some general observations on 
population pressure here to help place the demographic detail of Chapter 5 in perspective. 

It is important to understand, first, that population pressure may arise from large absolute 
population size or from unsuitable geographical distribution of population 


or from high rates of change in size or distribution or from a combination of these factors. 
Population pressures in Europe, Japan, and the United States, for example, are associated more 
with the large sizes those populations have already attained than with their rates of population 
growth, which are relatively low. Of course, even a low rate of growth in such a country is made 
more significant precisely because the absolute impact of population size and the impact per 
person are already so high there; but saying the pressures are primarily related to size is to 
recognize that those regions would be experiencing essentially similar difficulties even if their 
rates of population growth were zero at this time. 

In certain of the less developed countries, by contrast, population pressures are associated more 
with an inability of social and economic services to keep up with rapid population growth than 
with the absolute sizes of the populations. This is the situation in many countries in Latin 
America and Africa. The pressures are often exacerbated by high rates of change in population 
distribution — namely, floods of migrants into the cities from the countrysides. 

Many Asian countries illustrate a third situation, in which awesome pressures arise both from the 
great size the populations have already reached and from continuing high rates of growth. India, 
Pakistan, Bangladesh, and the People's Republic of China are examples of this general situation, 
although they differ from each other in important respects. 

A theme that emerges from these considerations and runs through Chapter 5 is that few 
generalizations about population and population pressure can be expected to hold worldwide. 
The variety of conditions and pressures in different countries is enormous. Understanding the 
overall demographic situation is possible only when these differences are examined in detail, and 
this we have undertaken to do. 

Chapter 6 and Chapter 7 deal with some of the resources necessary for the support of the human 
population — more specifically, those resources it has become customary to call renewable. They 
are land, soil, water, forests, and food. Whether renewable is really a useful generic label for 
these commodities is questionable. The planet's stores of land and water are essentially fixed, 
and they can be used over and over again; but the same can be said of metals, usually thought of 
as nonrenewable. The difference is only that, when metals are dispersed in use, society must 
write them off or pay a heavy price in energy to reconcentrate them, whereas when water is 
dispersed in use, solar energy reconcentrates it "free" through the hydrologic cycle. At the same 
time, it is easy to think of circumstances in which soil is a highly nonrenewable resource ~ for 
example, when it is eroded by wind or water much faster than natural processes can replenish it. 
If soil is lost, the capacity to grow food and forests in that region is lost, too, so those become, at 
least locally, nonrenewable. 

The resources of land, soil, water, and forests are so intimately related that we have put them 
together in Chapter 6. We devote more space in that chapter to the complex chemical and 
physical characteristics of soil than is customary in environmental science texts because we 
believe that soil's central role in support of human society has been widely underestimated. The 
complexity and subtlety of the 


functions performed by soil are a major reason that food production cannot be greatly expanded 
merely by planting crops on any reasonably flat space not otherwise occupied. That the soil is an 
ecosystem in itself, moreover, should serve as a warning that it may be vulnerable not only to the 
traditional threats of erosion, waterlogging, and salination but also to systematic disruption by 
the chemical assaults of industrial society. (These threats are considered in Chapter 11.) 

Food, the subject of Chapter 7, is simultaneously a biological, technological, economic, social, 
and political problem. Eventual limits to food production by conventional methods will be 
imposed by limitations on the resources of soil and water, discussed in Chapter 6, and of course 
by the availability of sunlight. Short of these resource limits, however, society may already be 
pushing up against limits of another kind — limits in its ability to make available the expensive 
technology of high-yield agriculture, to supply and pay for the industrial energy this technology 
requires on a continuing basis, and to absorb the environmental burdens that expanding 
agriculture by any means produces. The difficulties of growing enough food are compounded by 
the problems of distributing the food to those who are hungry. (We are regularly discouraged to 
hear discussions of food "surpluses" -meaning a supply that exceeds economic demand ~ when 
so much of the problem is that the people who are hungry are also broke.) Like so many of the 
problems treated in this book, the nature of the food problem varies enormously from region to 
region, which makes generalizations difficult. Nevertheless, we believe that at least one 
generalization emerges clearly from the detailed analysis in Chapter 7: concerning the future of 
food supply, none dare be complacent. 


The History and Future of the Human Population 

Prudent men should judge of future events by what has taken place in the past, and what is 
taking place in the present. 

-Miguel de Cervantes (1547-1616), Persiles and Sigismunda 

We shall see finally appear the miracle of an animal society, a complete and definitive ant-heap. 

- Paul Valery ( 1871-1945) 

The first small population of human beings probably appeared on Earth more than 2 million 
years ago on the continent of Africa. Since then, the human population has spread out to occupy 
virtually the entire land surface of the planet. And in the past century or two it has exploded in 
numbers. Today roughly between 4 and 5 percent of all the people who have ever lived inhabit 
Earth — some 4 billion. - 

This chapter recounts the history of population growth and explores its projected course in the 
future, especially the next century or so. Principles of human population dynamics (demography) 
are also presented here, with a view to explaining the present predicament of humanity and how 
the current demographic situation shapes the future and limits social options. Population size and 
growth have profound, if seldom noticed, effects on the course of events, and this is likely to be 
even more true in the future than it has been in the past. 


Since there are no substantial historical data on which to base estimates of population size and 
changes before 1650, estimates must be based on circumstantial evi- 

*N. Keyfitz, How many people have ever lived on Earth? Demography, vol. 3 ( 1966), pp. 581- 
582. Unless otherwise noted, current population statistics are based on the United Nations 
Concise report on the world population situation in 1970-1975 and its king-range 
implications, 1974; The Environmental Fund, Inc., 1975 World population estimates, 
Poputation Reference Bureau, 1975 Population data sheet; various annual volumes of the 
United Nations Demographic yearbook ( United Nations, New York) and Statistical abstract 
of the United States ( United States Department of Commerce; Washington, D.C.); and United 
States National Center for Health Statistics (USNCHS), Monthly vital statistics report of the 
United States. 
-181- PAGENUMBER182 




FIGURE 5-1 The growth of human numbers for the past half-million years. If the Old Stone Age 

were in scale, its baseline would extend about 1 8 feet to the left. (Adapted from Population 

bulletin, vol. 18, no. 1. Courtesy of the Population Reference Bureau, Inc., Washington, D.C.) 

dence. For instance, agriculture was unknown before about 8000 B.C.; prior to that all human 
groups made their living by hunting and gathering. No more than 52 million square kilometers 
(20 million square miles) of Earth's total land area of some 150 million square kilometers (58 
million square miles) could have been successfully utilized in this way by our early ancestors. 
From this and the population densities of hunting and gathering tribes of today, it has been 
estimated that the total human population of 8000 B.C. was about 5 million people. 

The sizes of population at various times from the onset of the Agricultural Revolution until the 
first scanty census data were recorded in the seventeenth century have also been estimated. This 
was done by extrapolation from census figures that exist for present-day agricultural societies 
and by examination of archaeological remains. Such data as the numbers of rooms in excavated 
ancient villages have proven especially useful for calculating village populations. It is thought 
that the total human population at the time of Christ was around 200 million to 300 million 
people, and that it had increased to about 500 million (half a billion) by 1650. It then doubled to 
1000 million (1 billion) around 1850, and doubled again to 2 billion by 1930. The course of 
human population growth is traced in Figure 5-1 . Note that the size of the population has (with a 
few setbacks) increased continuously and that the rate of increase has also risen. 

Perhaps the simplest way to describe the growth rate is in terms of doubling time — the time 
required for a population to double in size. In growing from 5 million in 8000 B.C. to 500 
million in 1650, the population increased a hundredfold. This required between six and seven 
doublings within 9000 or 10,000 years: 

Population (millions): 

5 -» 10 -» 20 -* 40 -+ 
12 3 4 

80 -» 160 -». 320 -» 640 
5 6 7 

Thus, on the average, the population doubled about once every 1500 years during that period. 
The next doubling, from 500 million to I billion, took 200 years; and the 


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on: Book Title: Ecoscience: Population, Resources, Environment. Contributors: Paul R. Ehrlich - author, Anne 
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material is protected by copyright and, with the exception of fair use, rp^rlloL be luiLtier copied, distributed 

rm or by any means. 

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FIGURE 5-2 Human population growth plotted on a log-log scale. Plotted in this way, 

population growth is seen as occurring in three surges, associated with the cultural, agricultural, 

and industrial-medical revolutions (discussed later in this chapter). (Adapted from Deevey, 


doubling from 1 billion to 2 billion took only 80 years. The population reached 4 billion around 
1975, having doubled again in only 45 years. The rate of growth in the early 1970s would, if 
continued, double the population in about 36 years. Table 5-1 summarizes human population 
history in these terms. 

To take a slightly different perspective, it took 1 million or 2 million years to achieve a 
population size of 1 billion around 1850. The next billion was added in 80 years ( 1850-1930). 
The third billion came along in only 30 years ( 1931-1960), and the fourth took only about 15 
years ( 1961-1975). If this rate of growth continued, the fifth billion would be added in just 
slightly more than a decade (by 1987). 

The sort of graph shown in Figure 5-1 does not reveal details of trends in the long, slow growth 
of the human population before the current millennium. But if population size and time are 
plotted against one another on logarithmic scales (a log-log graph, as in Figure 5-2 ) , a greater 
range of time can be shown, and more detail in the lower range of population sizes is revealed. 
Notice that the log-log graph shows three surges of population growth, one about 600,000 years 
ago, one about 8000 years ago, and one about 200 years ago. 

The reasons for the patterns that appear on both graphs are reasonably well understood, but 
before examining them, we must consider some aspects of human population dynamics. 

TABLE 5-1 

Doubling Times of the Human Population 

Time in which 

Estimated world 

population doubles 




8000 B.C. 

5 million 


1650 A.D. 

500 million 


1850 A.D. 

1000 million (1 billion) 


1930 A.D. 

2000 million (2 billion) 


1975 A.D. 

4000 million (4 billion) 
Computed doubling 

time around 1975 


Birth and Death Rates 

A human birth rate (b) is usually expressed as the number of births per 1000 persons per year, 
rather than per individual as in Chapter 4. The total number of births during the year is divided 
by the estimated population at the midpoint of the period. For example, in the United States there 
were 3,159,000 live births during the twelve months ending November 30, 1975. The population 
on May 30, 1975 (the midpoint of that period), was estimated to be 213,000,000. The birth rate 


TABLE 5-2 

Doubling Times at Various Rates 

of Increase 

Annual increase (%) 





Doubling time (years) 

(b) for that period was therefore 3,159,000/213,000,000 = 0.0148. There were 0.0148 births per 
person, or 0.0148 x 1000 = 14.8 births per 1000 people. Similarly, there were 1,912,000 deaths 
during the period, giving a death rate (d) of 1,912,000/213,000,000 = 0.0090 x 1000 = 9.0 deaths 
per 1000 people in the year from December 1, 1974 to November 30, 1975. - 

Growth Rate 

Ignoring migration, the growth rate (r) is calculated by subtracting the death rate from the birth 
rate (b - d). During the year ending November 30, 1975, the United States growth rate was 14.8 - 
9.0, or 5.8 per 1000. That is, in the period from December 1, 1974, to November 30, 1975, 5.8 
persons were added to each 1000 in the United States population. Technically, this is the rate of 
natural increase, since migration is ignored. To add to the confusion, demographers express this 
growth rate as a percent annual increase — that is, not as a rate per 1000 or per person, but as a 
rate per 100. In the example cited above, the annual increase would be 0.58 percent, a typical 
rate for an industrialized nation today. 

Population growth for the entire world presents a different picture. In the early 1970s the 
estimated world birth rate was around 32 per 1000 per year and the death rate about 13 per 1000. 
The growth rate was thus 32 - 13 = 19 per 1000, or approximately 1.9 percent. _This gives a 
finite rate of increase (k) of about 1.02 per year and a reasonably accurate estimate of r = 0.019 
(doubling time, 36 yr) as discussed in Chapter 4. The age composition of the entire human 
population is not stable, and the growth of that population therefore is only roughly exponential. 
If the vital rates of the early 1970s persisted, however, the population would grow to 8 billion 
people around the year 2010. Table 5-2 shows the doubling times that would be associated with 
various annual percentage increases. 

History of Population Growth 

The populations of our ancestors a few million years ago {Australopithecus and relatives) were 
confined to Africa and numbered perhaps 125,000 individuals. By that time, these ancestors of 
ours had already "invented" culture, the body of nongenetic information passed from generation 
to generation. The volume of culture is, of course, vastly greater today than in the days of 
Australopithecus . In those days human culture was transmitted orally and by demonstration from 
the older to the younger members of the group. It doubtless consisted of information about 
methods of hunting, gathering, and preparing food; rules of social conduct; identification of 
dangerous enemies; and the like. Today, of course, human culture includes information 
transmitted and stored in such diverse places as books, phonograph records, photographs, 
videotapes, and computer tapes. 

The possession of a substantial body of culture is what differentiates human beings from the 
other animals. During human evolutionary history the possession of culture has been responsible 
for a great increase in human brain size (the australopithecines had small brains, with an average 
volume of only about 500 cubic centimeters). Early human beings added to the store of cultural 
information, developing and learning techniques of social organization and group and individual 
survival. This gave a selective advantage to individuals with the large brain capacity necessary to 
take full advantage of the culture. Larger brains in turn increased the potential store of cultural 

information, and a selfreinforcing coupling of the growth of culture and brain size resulted. This 
trend continued until perhaps 200,000 

2 USWCHS, Vital statistics, vol. 24, no. 1 1 ( February 2, 1976). 

The Population Reference Bureau estimate for that period was b = 31.5 per 1000, d = 12.8 per 
1000, r = 18.7 per 1000, or an increase of slightly less than 1.9 percent. Other sources ( Lester 
R. Brown, World population trends: Signs of hope, signs of stress) have postulated lower rates 
for 1975: b = 28.3, d= 11.9, r = 16.4 per 1000, or 1.64 percent per year — a substantial 
decline since 1970. 

years ago, when growth of brain size leveled off at an average of some 1350 cubic centimeters, 
and human beings considered to belong to the same species as modern humanity, Homo sapiens, 

The evolution of culture had some important side effects. Although the prehistoric human birth 
rate probably remained around 40 or 50 per 1000, cultural advances probably caused a slight 
decline in the average death rate. But the average death rate could not have been more than 0.004 
per 1000 below the birth rate (the corresponding growth rate being 0.0004 percent or less), and 
there unquestionably were sizable fluctuations in birth rates and, especially, in death rates, 
particularly during the difficult times associated with glacial advance. Long before the 
Agricultural Revolution, humanity had spread out from Africa to occupy virtually the entire 
planet. It is known that human beings reached the Western Hemisphere some time before 45,000 
B.C. As they became ubiquitous, increased hunting and gathering efficiency may have led, 
among other things, to the extinction of many large mammals, such as the great ground sloths, 
saber-toothed tigers, and woolly mammoths. - 

The Agricultural Revolution. The consequences of cultural evolution for human population 
size and for the environment were minor compared with those that were to follow the 
Agricultural Revolution. It is not certain when the first group of Homo sapiens started to 
supplement its hunting and food-gathering with primitive farming. Archeological studies have 
produced firm evidence that village-farming communities functioned in the Middle East between 
7000 and 5500 B.C. ^_Around that time certain groups of people in the hills flanking the Fertile 
Crescent, in what is now the border area of Iraq and Iran, gradually began to take up a new mode 
of life, cultivating a variety of crops and domesticating edible animals. Those people had 
previously practiced intensive food collection, as do Eskimos today, and presumably they were 
intimately familiar with the local flora and fauna. It would have been a natural step from 
gathering food to producing it. 

With the beginnings of agriculture, growth of the human population started to accelerate. Two 
general explanations have been offered. The conventional explanation of this agriculture-related 
increase until recently was that the sizes of earlier populations had been kept in check by high 
natural mortality and that the development of agriculture tended to reduce the death rate. 
Agriculture not only allowed production of food to replace the constant search for it, it also 
permitted people to settle in one place. This in turn generated possibilities for storing vegetable 

foods in granaries and bins, and meat on the hoof. Farmers were able to feed more than their own 
families, and as a result some members of early agricultural communities were able to turn 
entirely to other activities. All of these changes helped to raise the general standard of life. 
Wheeled vehicles appeared; copper, tin, and then iron were utilized; and dramatic sociopolitical 
changes occurred, along with urbanization. Human existence thus began to lose some of its 
hazards, and the average life expectancy began to creep upward from its primitive level of 
perhaps twenty to twenty-five years. - 

Recently, a rather different view of population growth just after the Agricultural Revolution has 
developed. It does not dispute the historical trends already described, but it does dispute that 
their primary cause was a lowered death rate. _Instead, the observed changes are attributed 
primarily to changes in the birth rate. 

Using evidence from living populations of hunters and gatherers such as the !Kung _bushmen of 
southern Africa, from the demography of extinct human populations inferred from fossil 
samples, and from the behavior of nonhuman primates, supporters of this view have concluded 
that our ancestral hunters and gatherers consciously tried to space the births of their children. If 
this is true, one of the most "human" of all behavioral traits — the conscious control of 
reproduction — at least with respect to spacing, if not of total family size — may 

P. S. Martin and H. E. Wright, eds., Pleistocene extinctions; P. S. Martin , Pleistocene niches 
for alien animals; C. A. Reed, Extinction of mammalian megafauna in the Old World late 
quaternary. See also Chapter 4. 
5 R. J. Braidwood, The Agricultural Revolution; Wilhelm G. Solheim, II , "An earlier 

Agricultural Revolution", Scientific American, April 1972, pp. 34-41. 
For example, J. D. Durand, A long-range view of world population growth. 



See D. E. Dumond, The limitation of human population: a natural history, for a summary. 
The exclamation point denotes a click made with the tongue against the roof of the mouth, a 
sound with no counterpart in English. 
-185- PAGENUMBER186 

be many thousands of years older than was previously thought. Child-spacing (with gaps of at 
least three to five years) among nomadic hunters and gatherers was presumably necessitated 
partly by the inability of the mother to carry more than one child. Another likely reason would 
have been the need to nurse each child for at least three years because the environment lacked 
the soft food required for earlier weaning. ^Moreover, it appears likely that prolonged breast- 
feeding itself may help in suppressing fertility. Infanticide may also have been practiced widely 
among hunter-gatherers, when children were born too soon and in times of scarcity. — 

Anthropological research on various contemporary and recent hunter-gatherer groups (such as 
the !Kung bushmen) and on migratory agriculturalists such as the Polynesians indicates that 
those people have been quite conscious of population pressures and aware of the reasons behind 
many of their customs relating to marriage and child-bearing. The Polynesians are renowned for 
their long-distance migrations, an obvious response to population pressure. But they also 
practiced infanticide, polyandry, nonmarriage of landless younger sons, abstention from sexual 

activity for a period of time after birth of a child, coitus interruptus, and abortion. ^Then- 
awareness of the need to limit their numbers probably was sharpened by their confinement to 
small islands. Whether most hunter-gatherers have been conscious of a need to limit their final 
family sizes is problematical; but attention appears mainly to have focused on child-spacing. 

The imperatives for child-spacing experienced by hunter-gatherers apparently disappeared when 
agriculture began. Although some changes at that juncture worked toward slowing population 
growth (for example, unlike hunters and gatherers, a significant proportion of the people in many 
agrarian societies remains unmarried it is generally agreed that agriculture and high natality go 
hand in hand. The prime reason for this, many demographers believe, is the perceived economic 
value of children to farming families. (This pronatalist attitude may still be a major factor in 
keeping birth rates high in peasant communities today.) The increased natality in agrarian 
societies, not a decline in death rates, is considered in this school of thought to have been the 
main reason for the acceleration of population growth following the Agricultural Revolution. 
They claim that mortality from disease probably increased with the greater population densities 
of agrarian societies, especially in preindustrial cities, and that there is no persuasive evidence 
that death rates declined with the start of farming. 

Because of the uncertainty of early demographic records and especially the problems of 
reconstructing prehistoric vital rates, it is difficult to evaluate the two explanations of the 
population explosion that followed the Agricultural Revolution. It seems clear, however, that the 
earlier "lowered-death-rate" explanation was at best oversimplified. 

Population growth after the Agricultural Revolution. The growth of human populations was 
not continuous after the Agricultural Revolution. Civilizations rose, flourished, and 
disintegrated; periods of good and bad weather occurred; and those apocalyptic horsemen — 
pestilence, famine, and war — took their toll. Of course, there has been no accurate record of 
human population sizes until quite recently, and even today demographic statistics for many 
areas are unreliable. A general picture that is quite adequate for the purposes of our discussion 
can be reconstructed, however. Although the global trend (indicated in Figure 5-1 ) has been one 
of accelerating increase, a great many local population explosions and crashes are concealed in 
that trend. For example, bubonic plague (black death) killed an estimated 25 percent of the 
inhabitants of Europe between 1348 and 1350. From 1348 to 1379 England's total population 
was reduced almost by half, from an estimated 3.8 million to 2.1 million. Many cities lost half or 

more of their inhabitants in the second half of the fourteenth century. The demographic effect 

of repeated visitations of plague on the European population 

Mildred Dickeman, Demographic consequences of infanticide in man. 
1 Barnes T. Tanner, "Population limitation today and in ancient Polynesia", BioScience, vol. 25, 
no. 8 ( August 1975), pp. 513-516. 

1 9 

W. L. Langer, The black death. 

J. B. Birdsell, Some predictions for the Pleistocene based on equilibrium systems among 

recent hunter-gatherers; R. B. Lee, Population growth and the beginnings of sedentary life 

among the IKung bushmen; G. B. Kolata, IKung hunter-gatherers: Feminism, diet, and birth 











— „ 


J 1 1 

1 1 

1 1 






FIGURE 5-3 The effects of the bubonic plague on the size of the European population in the 

fourteenth and seventeenth centuries. The curve is an estimate based on historical accounts; 

actual data are scarce. (Adapted from Langer, 1964.) 

is represented in Figure 5-3 . The plague also triggered great social unrest, a typical concomitant 
of both dramatic increases and reductions in population size. The social and psychological 
effects lingered for centuries. 13 These plagues were only the most dramatic and wellknown 
epidemic in history. Numerous other plagues have taken their tolls over the centuries. For 
example, the influenza epidemic of 1917/ 1918, a much milder disease, killed some 20 million 
people in one year. 

Famine has also been an important recurring contributor to high death rates. Floods, drought, 
insect plagues, warfare, and other disasters often have pushed populations over the thin line 
between subsistence and famine. One study by Cornelius Walford _^_lists more than 200 famines 
in Great Britain between 10 A.D. and 1846 A.D. ( Box 5-1 ). Many, of course, were local affairs, 
which nevertheless could be very serious in the absence of efficient transport systems for 
transferring surplus food from other areas. Another study counts 1828 Chinese famines in the 
2019 years preceding 191 1, a rate of almost one per year. Some of those famines and some in 
India have been known to result in millions of deaths. Even in this century famine has killed 
many millions. For example, perhaps from 5 million to 10 million deaths have been attributed to 
starvation in Russia in the first third of the century ( 1918-1922, 1932-1934). Perhaps as many as 
4 million deaths were caused by famine in China ( 1920/ 1921) before the revolution in 1949, 
and unknown millions in the first decade afterward. And between 2 and 4 million deaths were 
caused by famine in West Bengal, India, in 1943. 

Warfare has often created conditions in which both pestilence and famine thrived, but it is 
difficult to estimate the direct consequences of war for population size. In many areas of the 
world, wars must have made a major contribution to the death rate, even when the conflict was 
between preagri cultural groups. The effect of warfare on the sizes and distribution of populations 
in New Guinea, for example, was rather dramatic until quite recently. For security, villages in 
many areas were situated exclusively on hilltops, some of them thousands of feet above the 
nearest available water. A likely fate for a New Guinean man, woman, or child was death at the 
hands of a hostile group. 

Throughout the history of Western civilization, war has been essentially continuous. This 
doubtless helped to maintain high death rates, particularly by creating food shortages and the 
preconditions of plagues. Barbarian invasions of the Roman Empire ( 375-568 A.D.), the 
Hundred Years' War (1337-1453), and especially the Thirty Years' War (1618-1648) caused 
substantial increases in the death rate in Europe. To give a single instance from the last of those 
conflicts, the storming and pillaging of Magdeburg by Catholic forces in 1631 caused an 
estimated 20,000 deaths. Indeed, some historians feel that as many as a third of the inhabitants of 
Germany and Bohemia died as a direct result of the Thirty Years' War. 

The Peace of Westphalia ended the Thirty Years' War in 1648, introducing a period of relative 
tranquility and stability. At that time the Commercial Revolution was in full swing. Power was 
concentrated in monarchies, after having been decentralized in a loose feudal structure, and 
mercantilism was the economic order of the day. Perhaps 

1 3 

W. L. Langer, The next assignment. 
The famines of the world: Past and present. 
-187- PAGENUMBER188 

BOX 5-1 Famines 

A small sampling of quotes from Cornelius Walford 's 1878 chronology of 350 famines will give 
some feel for the ubiquity in time and space of this kind of catastrophe. (Quotation marks 
indicate where Walford was quoting directly from his sources.) Those who have seen films of 
recent famines in India and Africa are unlikely to consider scenes like those described below as 
things of the past. 

B.C. 436 Rome. Famine. Thousands threw themselves into the Tiber. 

A.D. 160 England. Multitudes starved. 

192 Ireland. General scarcity; bad harvest; mortality and emigration, "so the lands and houses, 
territories and tribes, were emptied." 

33 lAntioch. This city was afflicted by so terrible a famine that a bushel of wheat was sold for 
400 pieces of silver. During this grievous disaster Constantine sent to the Bishop 30,000 bushels 

of corn, besides an immense quantity of all kinds of provisions, to be distributed among the 
ecclesiastics, widows, orphans, etc. 

695-700 England and Ireland Famine and pestilence during three years "so that men ate each 

1 193-1 196 England, France. "Famine occasioned by incessant rains. The common people 
perished everywhere for lack of food." 

1299 Russia. Ravaged by famine and pestilence. 

1412-1413 India. Great drought, followed by famine, occurred in the GangesJumna delta. 

1600 Russia. Famine and plague of which 500,000 died. 

1769-1770 India. (Hindustan) First great Indian famine of which we have a record. It was 
estimated that 3,000,000 people perished. The air was so infected by the noxious effluvia of dead 
bodies that it was scarcely possible to stir abroad without perceiving it; and without hearing also 
the frantic cries of victims of famine who were seen at every stage of suffering and death. 

1770 Bohemia. Famine and pestilence said to carry off 168,000 persons. 

1789 France. Grievous famine; province of Rouen. 

1790 India. Famine in district of Barda ... so great was the distress that many people fled to 
other districts in search of food; while others destroyed themselves, and some killed their 
children and lived on their flesh. 

1877-1878 North China. "Appalling famine raging throughout four provinces (of) North China. 
Nine million people reported destitute, children daily sold in markets for (raising means to 
procure) food. . . . Total population of districts affected, 70 millions. . . ." The people's faces are 
black with hunger; they are dying by thousands upon thousands. Women and girls and boys are 
openly offered for sale to any chance wayfarer. When I left the country, a respectable married 
woman could be easily bought for six dollars, and a little girl for two. In cases, however, where it 
was found impossible to dispose of their children, parents have been known to kill them sooner 
than witness their prolonged suffering, in many instances throwing themselves afterwards down 
wells, or committing suicide by arsenic. 

1878 Morocco. ". . . If you could see the terrible scenes of misery—poor starving mothers 
breaking and pounding up bones they find in the streets, and giving them to their famished 
children ~ it would make your heart ache." 

-188- PAGENUMBER189 

the most basic new idea of mercantilism was that of government intervention to increase the 
power of the state and (more important, from our point of view) the prosperity of the nation. 
Planning by government was extended to provide economic necessities for the population. 

The preindustrial rise of population, 1650-1850. In the middle of the seventeenth century, a 
period of relative peace started in a postfeudal economic environment. Simultaneously, a 
revolution in European agriculture—a revolution that was largely a result of the Commercial 
Revolution—began to gather momentum. It accelerated rapidly in the eighteenth century. Rising 
prices and increasing demand from the growing cities added to the commercial attractiveness of 
farming. The breakdown of the feudal system gradually destroyed the manorial estates. On those 
estates each male serf had assigned to him several scattered strips of land, which were farmed 
communally. The serfs grew increasingly unhappy with the communal farming, so the strips 
were rearranged into single compacted holdings leased by individual peasants from the 

As landowners wished to put more land to the plow, they began to enclose old communal 
woodland and grazing lands with hedges and walls, barring the peasants from resources essential 
to their subsistence. This movement was especially pronounced in Great Britain, where it was 
promoted by a series of special acts of Parliament. Furthermore, much of the peasantry was 
dispossessed or forced out of agriculture by competition from the more efficient large farming 
operations. Agriculture was transformed into big business. 

Accompanying these changes were fundamental improvements in crops and farming techniques. 
For instance, the role of clover in renewing soil (by replacing lost nitrogen) was discovered in 
England by Lord Charles Townshend. This made the practice of letting fields lie fallow every 
third year unnecessary. Other improvements were made in methods of cultivation and in animal 
breeding. Agricultural output increased and so, consequently, did the margin over famine. It 
seems plausible that a combination of commercial and agricultural revolutions, a period of 
relative peace, and the disappearance of the black death all combined to reduce the death rate and 
produce the European population surge that started in the mid-seventeenth century. 15 Between 
1650 and 1750 it is estimated that the populations of Europe and Russia increased from 103 
million to 144 million. 

An additional factor that may have contributed to this acceleration in growth was the opening of 
the Western Hemisphere to European exploitation. In 1500 the ratio of people to available land 
in Europe was about 10 per square kilometer. The addition of the vast, virtually unpopulated 
frontiers of the New World reduced the ratio for Europe plus the Western Hemisphere to less 
than 2 per square kilometer. As historian Walter Prescott Webb wrote, this frontier was, in 
essence "a vast body of wealth without proprietors." ^_Thus, not only was land shortage in 
Europe in part alleviated, but several major European nations were enriched. Both factors 
encouraged population growth. 

Furthermore, the introduction of two new foods from the New World, maize (corn) and the 
potato, either of which could provide sustenance for a large family from a very small plot of 
land, may have fueled a population increase by expanding the feeding base. ^JVlaize was 

widespread in southern Europe by the eighteenth century, during which the populations of both 
Spain and Italy nearly doubled. 

Although we can speculate with ease about the causes of Europe's population boom between 
1650 and 1750, it is somewhat more difficult to explain a similar boom in Asia. The population 
there increased by some 50 to 75 percent in that period. In China, after the collapse of the Ming 
dynasty in 1644, political stability and the agricultural policies of the Manchu emperors 
doubtless led to a depression of death rates. The introduction of maize and peanuts to China in 
the eighteenth century may also have stimulated growth there, as the potato and maize seem to 
have done in Europe. 

15 The disappearance of black death was possibly due to the displacemeat of the black rat, which 
lived in houses by the sewer-loving brown rat. This lessened contact between people and rats, 
and thus reduced the chances of plague-carrying fleas reaching human beings. In London the 
great fire of 1666 destroyed much of the city, which consisted largely of rundown wooden 
buildings that provided excellent shelter for the rats. By orders of the king, the city was rebuilt 
with brick and stone, thus making it much more secure from plague. 
The great frontier . 

1 7 

W. L. Langer, American foods and Europe's population growth, 1750-1850. 

Much of the Asian population growth during that period probably took place in China, since 
India was in a period of economic and political instability caused by the disintegration of the 
Mogul Empire. When the last of the Mogul emperors, Aurangzeb, died in 1707, India was racked 
by war and famine. British soldier and statesman Robert Clive and the East India Company 
established British hegemony in India during the period between 1751 and 1761. At a time when 
China may have had the world's most advanced agricultural system under the efficient Manchu 
government, India was a battleground for the British and French. And the rapid increase in 
power of the British East India Company following the Peace of Paris in 1763 did not bring rapid 
relief. Indeed, in a famous famine in 1770, about one-third of the population of Bengal is reputed 
to have perished—a circumstance that did not prevent industrious agents from maintaining the 
East India Company's revenue from Bengal during that year! 

World population seems to have grown at a rate of about 0.3 percent per year between 1650 and 
1750. The rate increased to approximately 0.5 percent between 1750 and 1850, during which 
time the population of Europe doubled. Because what demographic records exist are scanty, 
fragmentary, and often anecdotal, the causes of this acceleration are not known and have become 
a subject of some debate. The prevailing view has been that, in Europe at least, the surge 

1 X 

occurred in response to a number of favorable changes that acted chiefly to reduce mortality. — 

The most important cause of lowered death rates in preindustrial Europe seems to have been a 
decline in mortality from infectious diseases, especially tuberculosis (at least in England) and 
smallpox (resulting from the introduction of smallpox inoculation). Demographic historians T. 
McKeown, R. G. Brown, and R. G. Record attribute this change to "environmental 
improvements"— primarily, increased food production resulting from continued advances in 

agricultural techniques. ^_P. E. Razzell has suggested that the adoption of habits of personal 
hygiene following the popular introduction of soap and cheap utensils and cotton clothing had 


more to do with the reduction of infectious diseases than did nutritional improvement. He also 

cites drainage and reclamation of land as possibly helping to reduce the incidence of malaria and 
various respiratory diseases. 

Another group of historians puts an emphasis on rising birth rates as an explanation of the burst 
of growth in Europe's population that began in the seventeenth century. Even though agricultural 
socioeconomic systems (such as that of eighteenth-century Europe) generally promote high birth 
rates, there is evidence that through most of history, farming populations have kept their natality 
below the maximum. Not the least of this evidence is the relatively low growth rate during the 


first 10,000 years of agriculture (less than 0.1 percent.) This suggests that there was a "reserve" 

of natality that could permit an increase when needed. Some historians think such a reserve may 


have helped fuel Europe's seventeenth-century growth spurt. The claim is that rising birth rates 

played an important role in accelerating this population growth and that the decline in death rates 
may not have been significant or consistent until around 1900. _ The latter suggestion is not 
supported, however, by evidence from several northwestern European countries for which data 
exist (see Figure 5-4 ). 

Historian William Langer believes an overriding factor was the widespread acceptance of the 
potato as a staple food in northern Europe during the eighteenth century, particularly among 
peasants trying to subsist on very small pieces of land. 24 His view is that, by providing relatively 
abundant, nutritious food for the poor, potato cultivation encouraged couples to marry younger 
and raise larger families. Hence, it stimulated higher birth rates as well as contributing to the 
improved nutrition that led to lower death rates from disease. The potato's role in generating the 
population explosion in Ireland during this period (the Irish increased from 3 million to 8 
million), followed by a crash when potato blight destroyed the crop in 1846 and 1847 (1 million 
died, and 2 

1 R 

A. J. Coale, The history of the human population. 

T. McKeown, R. G. Brown, R. G. Record, An interpretation of the modern rise of population 

in Europe. 


P. E. Razzell, An interpretation of the modern rise of population in Europe— a critique. 

9 1 

E. A. Wrigley, Population and history; Dumond, The limitation, pp. 718-719. 


R. Freedman, The sociology of human fertility : A trend report and bibliography; Wrigley, 
Population, p. 161 ff. 
Dumond, The limitation, p. 719. 
Langer, American foods. 

Questia Med 

5A P 

Publication Infp 

H. Ehriich - a 
Page Number; 
or transmitted i 

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rmation: Book Title: Ecoscience: Population, Resources, Environment. Contributors: Paul R. Ehriich - author, Anne 
jr, John P. Holdreft-author, Publisher: W. H. Freeman. Ptace of Publication: 9an Francisco. Publication Year: 1977. 
1. Tlni5rjiawftelj|*0n*fl?acted by copyright and, with the exception orrfPflBe, nS^y not be further copied, distributed 







erica, Inc. 

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Modern medicine — 

oLl I I I I I l l I I I I I I 1 I I I I I 

1751-61- 71- 81- -9I-I60J- 11- "21- "3»- *' 51 61 71 9i 9i 1901- n- ?i 31 «i 


FIGURE 5-4 Mean annual death rates in England and Wales, Sweden, France, Ireland, and 

Hungary from the time of their initial registration. The times when some factors thought to play 

important roles in reducing mortality were introduced are indicated, too. Note that modern 

medicine—as distinct from personal and public hygiene—became an important influence only 

quite recently. (Adapted from McKeown, Brown, and Record, 1972.) 

million emigrated), is common knowledge. Many other areas in Europe were also threatened by 
the blight, but because the people were less completely dependent on a potato monoculture, more 

Quite probably all these factors— prosperity, increased food supplies, diminished mortality from 
disease, improved sanitation— contributed in some measure to the acceleration in Europe's 
population growth between 1750 and 1850. There is little evidence to support the hypothesis that 
birth rates rose significantly. Indeed, in some countries they were declining well before 1800. 
The argument that the acceleration of growth that accompanied the beginning of agriculture was 
primarily caused by increased natality seems to us persuasive; the claim that a significant trend 
in declining death rates did not develop until the end of the nineteenth century is less so, 
especially since considerable evidence exists to the contrary. 

Regardless of whether the growth in Europe's population between 1750 and 1850 was caused 
mainly by declining death rates, rising birth rates, or some combination of the two, twice as 
many people were living there at the end of the century as had been at the beginning. Moreover, 
this increase occurred despite several factors operating against it, notably a high rate of celibacy 

(nonmarriage) and the apparently widespread practice of infanticide. Reports of infanticide in 

Europe extend back into the Middle Ages, 26 but by 1800 in England, France, and Germany it 
seems to have reached almost epidemic proportions. The vast majority of the unwanted children 
were born to the poor— often to unmarried domestic servants and others unable to care for them. 
In this connection, there seems to have been considerable social and economic pressure against 
marriage for the landless poor, and even for younger sons of the wealthier classes. In London and 
Paris— and perhaps other cities, as well— foundling hospitals were established to care for the 
growing numbers of abandoned babies. But the level of care provided was so poor that the main 

result was the disappearance of infant bodies from streets, roadsides, and streams (evidently they 


were a very common sight before 1800). The horrors of widespread infanticide-largely 

suppressed and forgotten today—apparently per- 

W. L. Langer, Checks on population growth: 1750-1850. 
26 Dickernan, "Demographic consequences; Richard Trexler, Infanticide in Florence: New 
sources and first results, and The foundlings of Florence, 1395-1455", History of childhood 
quarterly, vol. 1 ( 1973), no. 1 (spring), pp. 98-1 16, and no. 2 (fall), pp. 259-284. 
Langer, Checks. 

sisted as late as 1 870—perhaps not coincidentally, about the time the birth-control movement was 
beginning (see Chapter 13). One might speculate that both the birthcontrol movement and 
Victorian prudery (especially the powerful disapproval of extramarital sexual activity and 
illegitimacy) were both reactions to infanticide. It is ironic, if so, that the same prudery for 
decades hindered the spread of birth control! 

The doubling of Europe's population between 1750 and 1850 also was achieved in the face of 
substantial emigration to the New World, where the population jumped from 12 million to about 
60 million in the same period. The United States grew from about 4 million in 1790, when the 
first census was taken, to 23 million in 1850. Much of the increase was due to immigration, 
mainly from England, but birth rates were also high (an estimated 55 per 1000 in 1800, declining 


to 43.3 per 1000 in 1850). Mortality data are lacking, but it is probable that the death rate in 

the United States was declining as it was in Europe, and for similar reasons. As just one example, 
mass inoculation against smallpox was first introduced and shown to be effective during the 
Revolutionary War. 

Growth in Asia between 1750 and 1850 was slower than in Europe, amounting to an increase of 
about 50 percent. Most of the developments that favored a rapid increase in Europe's population 
were to appear in Asia only much later, if at all. Little is known about the sizes of past 
populations in Africa, a continent that remained virtually unknown to the outside world until 
well past the middle of the nineteenth century. It is generally accepted that the population there 
remained more or less constant at around 100 million, plus or minus 5 or 10 million, between 
1650 and 1850. After that, death rates appear to have declined somewhat, perhaps because of 
improved food supplies and distribution and the introduction of European sanitation and 

The demographic transition, 1850-1930. Death rates in Europe and North America continued 
to decline during the period between 1850 and 1900, probably as a result of improvements in 
living conditions that accompanied the Industrial Revolution. Although the horrible conditions 
that prevailed in mines and factories during the early stages of the rise of industry are well 
known to all who have read the literature of the period, the overall situation in areas undergoing 
industrialization actually improved. Life in the rat-infested cities and rural slums of preindustrial 
Europe had been grim almost beyond description. Advances in agriculture, industry, and 
transportation by 1850 had substantially bettered the lot of the average person in Europe and 

North America. Improved agriculture had reduced the chances of crop failures and famine. 
Mechanized land and sea transport had made local famines less disastrous by providing access to 
distant resources. Great improvements in sanitation—notably, the installation of sewage systems 
and the purification of water supplies—in the last third of the nineteenth century helped to reduce 
death rates further. So did discovery of the role of bacteria in infection and the introduction of 
inoculation against disease. (The medical advances, however, appear to have had less impact on 
mortality than has traditionally been attributed to them.) — 

European death rates, which in 1850 had been in the vicinity of 22 to 24 per 1000, decreased to 
around 18 to 20 per 1000 by 1900 and went as low as 16 per 1000 in some countries. Combined 
rates for Denmark, Norway, and Sweden dropped from about 20 per 1000 in 1850 to 16 per 1000 


in 1900. In western and northern Europe in the latter half of the nineteenth century, low death 

"3 1 

rates (and the resultant high rate of population increase) helped stimulate massive emigration. — 

By the end of the century, another significant trend appeared: birth rates in several Western 
countries, including the United States, were also declining. In Denmark, Norway, and Sweden, 
the combined birth rate was around 32 per 1000 in 1850; by 1900 it had decreased to 28. Similar 
declines occurred elsewhere. This was the start of the so-called demographic transition— a falling 
of birth rates that followed declining death rates in Europe 

Historical statistics of the United States: Colonial times to 1957, U.S. Bureau of the Census, 
Washington, D. C. 


McKeown, Brown, and Record (An interpretation) are of the opinion that sanitation was far 
more important than medicine in reducing death rates before 1935. They point out that no 
significant drugs effective against infectious disease existed until sulfa drugs were developed 
in the 1930s and modern antibiotics in the 1940s. 

A. J. Coale, The decline of fertility in Europe from the French Revolution to World War II, in 
Fertility and family planning: A world view, S. J. Behrman; L. Corsa, Jr.; and R. Freedman, 

K. Davis, The migrations of human populations. 
-192- PAGENUMBER193 

and North America, which has generally been associated with industrialization. Figure 5-5 
shows the progress of the demographic transition in several European countries. 

What caused the decline in birth rates in the industrializing Western countries? No one knows for 
certain, but some rather good guesses have been made. In general, it has been assumed that the 
changed conditions of life caused by industrialization led to changed attitudes toward family 
size. In nineteenth-century western Europe, customs of late marriage (which reduces birth rates 
by reducing the number of years each woman is reproductively active) and a high proportion of 
nonmarriage were already established. Hence, declining birth rates presumably were due mainly 
to conscious and increasing limitation of the number of children born within marriage. 

It has been observed that in agrarian societies children are commonly viewed as economic 
bonuses. They serve as extra hands on the farm and take care of parents in their old age. This 


pronatalist point of view was beautifully expressed by Thomas Cooper in 1794. He wrote: "In 

America, particularly out of the large towns, no man of moderate desires feels anxious about a 
family. In the country, where dwells the mass of the people, every man feels the increase of his 
family to be the increase of his riches: and no farmer doubts about the facility of providing for 
his children as comfortably as they have lived. ..." 

As a society industrializes, the theory goes, these things change. Children are no longer potential 
producers; they become consumers, requiring expensive feeding and education. This is 
especially so once child labor is abolished and education becomes compulsory. Large families, 
which become more likely with lowered death rates, tend to reduce mobility and to make the 
accumulation of capital more difficult. Other factors affecting family size that have been 
mentioned are the education and employment of women, increased secularization of society, and 
the decline of traditional religious influence. — 

The demographic transition in Europe, however, was not limited to urban areas, although it may 
have begun there. Rapid population increase created a squeeze in rural areas as well, a squeeze 
that was compounded by the modernization of farms. A finite amount of land had to supply a 
livelihood for more people. As time went on, increased mechanization, which made larger farms 
more efficient and reduced the need for farm labor, made it more and more difficult for a young 
couple to establish themselves on a farm of their own. This may effectively have outweighed any 
traditional advantages of large farm families. As a result, rural birth rates also dropped, and many 
people moved to the cities seeking jobs in commerce and industry. 

Yet this traditional explanation of the demographic transition, in which industrialization is 
considered the prime influence, does not account for numerous exceptions. France, for example, 
industrialized late compared to some of its neighbors, yet it was the first country to experience 
declining birth rates, beginning in 1800 or earlier ( Figure 5-5 ). The United States also had a 
dropping birth rate well before industrialization had progressed very far; nor was land scarcity 
ever a significant factor here. 35 In England, the birthplace of the Industrial Revolution, on the 
other hand, the birth rate rose slightly between the 1840s and the 1870s, after which it declined 

Hungary, like most of eastern Europe, industrialized much later. Its demographic transition also 
began later (and proceeded faster), but the birth rate was dropping precipitously well before 
industrialization had taken place to any significant degree. 36 Thus it appears that, whereas 
industrialization (and accompanying urbanization and extension of compulsory education) may 
have been a factor in Europe's demographic transition, it clearly is not the whole explanation, 
and possibly is not even the main one. 

The twentieth century. The demographic transition in Western nations continued into the 
twentieth century. Yet, despite declining birth rates and a high rate of emigration from Europe, 
many of those countries were 

Coale, The history. 


Some information respecting America, p. 55. 

Coale, The Decline. 

Historical statistics of the United States. 
36 McKeown, Brown, and Record, An interpretation; M. S. Teitelbaum, Relevance of 

demographic transition theory for developing countries; Coale, Decline. 











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FIGURE 5-5 Mean annual birth and death rates in various nations for each decade for which 

statistics are available. The declines in birth rates began earliest in France and Sweden and most 

recently in Hungary, which had no significant industrial development before 1900. (Adapted 

from McKeown, Brown, and Record, 1972.) 









,' "'Errand and Wates 


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J I I I I I I I I I I I I I I I 

1BO0 1900 1950 

FIGURE 5-6 The growth of populations relative to their sizes in 1800. (Adapted from 

McKeown, Brown, and Record, 1972.) 

TABLE 5-3 

Populations, 1850 and 1950 (estimated, in millions) 

Asia Europe 

ind Asiai 

Source: United Nations ( 1963) and estimates (somewhat modified) by Willcox, Studies in 


Demography, and Carr- Saunders, World Population. 




and Asian 

























growing as rapidly as ever ( Figure 5-6 ). Of the four nations represented in Figure 5-6 , only 
Ireland lost population, and that was a direct result of the Irish potato famine in the 1840s. Birth 
rates there remained moderately high but were counterbalanced by heavy emigration. France 
maintained a very low rate of natural increase; Sweden and England both had relatively high 
ones, but Sweden's growth was much more depressed by emigration. The United States, by 

comparison, tripled its population between 1850 and 1900, spurred by massive immigration, 
even though its birth rate continued to decline steadily. 

The average growth rate of the world population between 1850 and 1950 was about 0.8 percent 
per year. Population increased in that time from slightly more than 1 billion to almost 2.5 billion. 
The estimated populations shown in Table 5-3 indicate that between 1850 and 1950 the 
population of Asia did not quite double, whereas it more than doubled in Europe and Africa, 
multiplied about fivefold in Latin America, and more than sixfold in North America. 

Toward the middle of the twentieth century, this pattern of relative growth rates began to change. 
By the 1930s declines in the birth rates in some European countries had outpaced declines in the 
death rates, and population growth had begun to slacken. By then the combined death rate of 
Denmark, Norway, and Sweden had declined to 12 per 1000, and the birth rate had dropped 
precipitously to about 16 ( Figure 5-4 ). North America followed a similar path, both in lower 
birth rates 




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FIGURE 5-7 Different patterns of change in birth and death rates and the rate of natural increase. 
Death rates dropped gradually in Western industrial countries such as Sweden and precipitously 
in LDCs such as Sri Lanka (Ceylon) and Mexico. (Courtesy of the Population Reference Bureau, 

Inc., Washington, D.C.) 

and in dramatically reduced immigration after the 1920s. 

Stimulated by improving economic conditions and World War II, however, birth rates rose again 
during the 1 940s and remained above replacement level in most developed countries until the 
late 1960s. Consequently, European growth rates generally averaged between 0.5 and 1.0 percent 
from 1945 to 1970. Since then, the decline in fertility has resumed, and a few countries had 
reached zero population growth (ZPG) by the mid-1970s. 

As population growth in industrialized nations slowed in response to low birth rates around the 
time of World War II, a dramatic decline in previously quite high death rates occurred in the 
nonindustrial nations. In some countries, such as Mexico, the decline started before the war. In 
others, such as Ceylon (now Sri Lanka), it did not start until the end of the war. Compare, for 
instance, the trend in Sweden since 1860 with that in Mexico since 1930 ( Figure 5-7 ) . This 
decline in the death rate was caused primarily by the rapid export of modern drugs and public- 
health measures from the developed countries to the less developed countries. The consequent 
"death control" produced the most rapid, widespread change known in the history of human 
population dynamics: the postwar population explosion. — 

The power of exported death control can be seen by examining the classic case of Ceylon's 
assault on malaria after World War II. Between 1933 and 1942 the death rate due directly to 
malaria in Ceylon was reported as about 2 per 1000. That rate, however, represented only a 
fraction of the malaria deaths, as many were reported as being caused by "pyrexia," a fancy name 
for fever. Actually, in 1934/ 1935 a malaria epidemic may have been directly responsible for 
fully half the deaths on the island. The death rate for those years rose to 34 per 1000. In addition, 
malaria, which infected a large portion of the population, made many people susceptible to other 
diseases and thus contributed to the death rate indirectly, as well as directly. 

The death rate in Ceylon in 1945 was 22 per 1000. The introduction of the insecticide DDT in 
1946 brought rapid control over the mosquitoes that carry malaria. Subsequently, the death rate 
on the island dropped 34 


Change in Age-Specific Death Rates of Males in Two LDCs 

1950-1952 rate as a 
percentage of 1920-1922 rate 





























Source: K. 
the world's 

Davis, Th 

e population 

impact on 

children in 

percent between 1946 and 1947, declined by about 50 percent of the 1945 level by 1955, and has 
continued to decline since then. In 1975 it stood at 8 per 1000. Although part of the drop was 
certainly due to other public health measures and the control of other diseases, much of it can be 
accounted for by the control of malaria. 

Victory over malaria, yellow fever, smallpox, cholera, tuberculosis, and other infectious diseases 
has been responsible for similar abrupt drops in death rates throughout the nonindustrial world. 
The decline has been most pronounced among children and young adults. They are the people 
most vulnerable to infectious diseases—the diseases most efficiently controlled by modern 
medical and public-health procedures. (Congenital problems in infants and degenerative diseases 
of old people reduce the proportionate effects of infectious disease in those age brackets.) The 
differential reduction of mortality can be seen clearly in data from Jamaica and Ceylon ( Table 

In the decade between 1940 and 1950, death rates declined 46 percent in Puerto Rico, 43 percent 
in Taiwan, and 23 percent in Jamaica. In a sample of eighteen less developed areas, the average 


decline in death rate between 1945 and 1950 was 24 percent. Figure 5-8 shows the dramatic 
change in death rates from the 1945 to 1949 average to the 1960/ 1961 average in selected Asian 

K. Davis, The amazing decline of mortality in underdeveloped areas. 


Changes in death rates in selected Asian nations. The average rates between 1945 and 1949 are 

compared with those of 1960 and 1961. (Adapted from Population bulletin, vol. 20, no. 2. 
Courtesy Population Reference Bureau, Inc., Washington, D.C.) 

A critical point to remember is that this precipitous decline in death rate is different in kind from 
the long-term slow decline that occurred throughout most of the world following the Agricultural 
Revolution. It is also different in kind from the comparatively faster decline in death rates in the 
Western world since 1650, even though both were achieved primarily through reduction in 
deaths from infectious diseases. The difference is that the plummeting death rates of less 
developed countries are a response to a spectacular environmental change in the LDCs, not to a 
fundamental change in their institutions or general ways of life. Furthermore, that change did not 
originate within those countries but was introduced from the outside. 

The factors that have been associated with a demographic transition (to lowered birth rates) in 
the developed countries therefore were not and still are not present in most non-Western 
countries, and birth rates there accordingly have remained high. For instance, the Indian birth 
rate in 1891 was estimated to be 49 per 1000 per year; in 1931 it was 46 per 1000, and in 1972 it 
was still around 40 per 1000, despite efforts to reduce it during the 1960s and 1970s (see Chapter 
13). And much of that small decline can be attributed to changes in the nation's age composition 
(see Box 5-4). 

In the decade 1930 to 1940, the populations grew in North America and Europe at 0.7 percent 
per year, whereas those of Asia grew at 1.1 percent, Africa at 1.5 percent, and Latin America at 
2.0 percent, even though death rates were still considerably higher in the last three areas 
mentioned. The annual world growth rate for the decade was 1.1 percent. 

Since then, the majority of the human population has moved rapidly from a situation of high 
birth and death rates to one of high birth rates and low death rates. The average annual world 
growth rate for the decade 1940 to 1950, presumably depressed by World War II, was 0.9 
percent (doubling time, 77 years). It then zoomed to 1.8 percent (doubling time, 38 years) from 
1950 to 1960 following the introduction of death control to the less developed regions. During 
the 1960s the world growth rate fluctuated between 1.8 and 2.0 percent per year. The entire 
population grew from about 2.3 billion in 1940 to 2.5 billion in 1950, to 3.0 billion in 1960, and 
3.6 billion in 1970. — 

According to the Population Reference Bureau, whose figures are based mainly on United 
Nations estimates, the world's population size in mid- 1976 was 4,019 million, the annual growth 
rate was 1.8 percent, and the doubling time 38 years. ^_These numbers, however, conceal very 
large differences in rates of growth among nations and regions, which are described in Box 5-2 
and summarized in Table 5-5 . (Appendix 1 presents detailed population estimates for 1976, 
prepared by the Population Reference Bureau.) 

Most LDC population figures, it should be noted, are only approximations because census data 
from many of them are inadequate or nonexistent. There is considerable controversy, for 
example, about the actual size and rate of growth of the Chinese population. China itself claimed 
a population of 800 million in late 1974. The United Nations put the population at 838 million in 
mid- 1975 and the growth rate at 1.7 percent. Demogra- 


United Nations Statistical warbook, 1973, and various other sources. 
1976 World Population Data sheet. 

pher Robert Cook of the Environmental Fund is skeptical of both estimates, and he places the 
1975 total at more than 900 million and the growth rate close to 2 percent. 41 (According to his 
figures, the world population in 1975 was growing at a rate of 2.2 percent per year and doubling 
in 32 years.) Another demographer, John S. Aird, of the United States Department of Commerce, 
has claimed that China's population exceeded 930 million in 1972 and was growing at 2.4 

At the other extreme, R. D. Ravenholt, director of the population program for the U.S. Agency 
for International Development (AID), claims that world population growth reached a peak 

between 1965 and 1970 and has now begun to decline. He places China's 1975 population at 

876 million and its growth rate at 0.8 percent. Ravenholt also offers lower estimates than Cook's 
of birth rates for many LDCs with family-planning programs. Obviously, large discrepancies in 
estimates for China alone can induce considerable variation in estimates for the world as a 
whole. Table 5-6 shows the differences between three of these estimates of world population size 
and growth. 

The most recent U.N. and U.S. government figures indicate that Ravenholt may be correct that 
the rate of population growth worldwide has begun to slacken since 1970. 42a Lester Brown of 
Worldwatch Institute attributes the slowdown to (1) unexpectedly successful results of China's 
population policies and a correspondingly lower death rate and, especially, birth rate there; (2) a 
significant decline in birth rates in nearly all developed countries; (3) some success in lowering 
birth rates in some LDCs; and (4) a rise in death rates in several LDCs, especially in South Asia, 
caused primarily by food shortages in the early 1970s (see Chapter 7). To the extent that lowered 
birth rates may be slowing population growth, this is encouraging news. To the extent that 
reduced growth is caused by a rise in death rates, it is a tragic confirmation of the often-ignored 
warnings of ecologists and others. 

TABLE 5-5 

Populations, 1960, 1970, and 1975, and Rates of Growth in Major Areas and Regions of the 


Annual growth 
rate (%) 

Population (millions) 






























More developed regions - 

Less developed regions 
Eastern Europe - 97 103 106 0.62 0.64 

Annual growth 
rate (%) 


Northern Europe - 

Southern Europe - 

Western Europe - 


Australia and New Zealand 


Micronesia and Polynesia 

Eastern South Asia 

Middle South Asia 

Western South Asia 


Japan - 

Other East Asia 

Eastern Africa 

Middle Africa 

Northern Africa 

Southern Africa 

Western Africa 


Middle America 

Temperate South America - 

Tropical South America 

Population (millions) 


















The regions marked with asterisks are considered as "more developed," 
a demographic point of view. 

Source: United Nations, Concise Report. 












































































































World population estimate, 1975. 

Gaining ground on the population front. 

Summarized in Lester R. Brown, World population trends. See also J. W. Brackett and R. T. 

Ravenholt, World fertility, 1976: An analysis of data sources and trends. 
-199- PAGENUMBER200 

BOX 5-2 Population Growth: 1975 to 2000 

By 1975 a striking disparity in population growth rates had arisen between the developed areas 
and the less developed areas of the world. Most DCs were growing very slowly, at less than 1 
percent per year; many LDCs were growing at 3 percent per year or more. At 3.5 percent a 
population would multiply more than thirty fold within a century. The world population in 1975 
was about 4 billion, and the growth rate in the early 1970s was 1.9 percent per year (doubling in 
36 years), according to the United Nations. _The following paragraphs outline the situation 
continent by continent. 

North America 

Canada and the United States together had a 1975 population of 237 million. A growth rate of 
0.9 percent per year gave a doubling time for the area of 77 years. Depending on future birth 
rates and migration patterns, North America could have as many as 350 million people in the 
year 2000, or it could have as few as 265 million. 

Latin America 

This area (the Western Hemisphere south of the United States) had a 1975 population of 326 
million and a growth rate of 2.7 percent, the highest rate for any major region. As a whole the 
population of the area was doubling every 26 years. Some Latin American countries have had 
extremely high growth rates and rapid doubling times in recent years. Honduras, for example, 
had a 1975 growth rate of 3.5 percent and a doubling time of 20 years. Doubling times for other 
representative countries were: Dominican Republic, 21 years; Mexico, 22 years; Peru, 24 years; 
Brazil, 25 years; Surinam, 27 years; Bolivia, 28 years; and Cuba, 35 years. A few countries in 
temperate Latin America were growing more slowly-Chile, Argentina, and Uruguay (doubling in 
38, 53, and 69 years, respectively). According to various projections, the population of Latin 
America in the year 2000 will be between 550 and 760 million (see Figure 5-9 ) . 


Europe is the demographic antithesis of Latin America. The 1975 European population 
(excluding European USSR and Turkey) was 474 million. The growth rate for the continent was 
0.6 percent, which gives a doubling time of 1 16 years. The more rapidly growing countries, such 
as Ireland, Spain, and Yugoslavia, had doubling times of between 60 and 80 years. But most 
European countries were doubling much more slowly than that: Czechoslovakia, every 116 
years; Italy, every 139 years; Hungary, every 173 years; and Austria, every 347 years. Both East 
Germany and West Germany have reached zero population growth. In the year 2000 Europe's 
population is projected to be about 540 million. 

The Soviet Union had a 1975 population of 255 million, a growth rate of 1.0 percent, and a 
doubling time of 70 years. For several years its growth rate has been similar to that of the United 
States, but it is now higher. The population is projected to reach 315 million by the year 2000. 


The 1975 population of Africa was 402 million people. Its current rate of growth is 2.6 percent, 
and its doubling time, 27 years. The pattern of growth is approaching that of Latin America, 
except that generally higher death rates in Africa result in a slightly lower growth rate. Sample 
doubling times were: Kenya, 21 years; Zambia, 22 years; Morocco, 24 years; Malagasy 
Republic, 24 years; Nigeria, 26 years; South Africa, 26 years; Egypt, 29 years; the People's 
Republic of Congo, 29 years; and Central African Republic, 33 years. Projections for the year 
2000, based on the assumption that the death rate will continue to decline, give Africa a 
population of between 730 and 900 million, second only to the total projected for Asia. Some 
specialists on Africa cite evidence that the relatively high death rates there will drop rapidly in 
coming years and the birth rates will remain high. If this occurs, growth rates of 3.5 percent or 
even 4 percent per year could become commonplace in Africa, in which case the population 
estimates given here would be too low. 

Concise report on the world population situation, 1970-1975, and its long-range implications. 
Doubling times that follow for regions and nations are from 1975 Population Data Sheet, 
Population Reference Bureau, Inc. 


* 4 

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Population growth in Latin America, 1920-2000. If fertility rates do not drop, the population of 

the area will undergo more than an eightfold increase in eighty years. (Adapted from Population 
bulletin, vol. 23, no. 3. Courtesy of the Population Reference Bureau, Inc., Washington, D.C.) 

Today's population giant, Asia, is inhabited by at least 2,273 million—over 2 billion—people. 
That figure— more than half the world's population-does not include the population of Asian 
USSR. It may be an understatement, moreover, if India's 1971 census results were incomplete 
and if some population estimates for China have been too low. Asia's 1975 growth rate was 2.1 
percent, and its doubling time was 33 years. Among the Asian nations only Japan shows a 
growth pattern similar to those of Europe and North America, although Hong Kong, Taiwan, 
Singapore, and South Korea are moving in that direction. Japan's growth rate in 1975 was 1.3 
percent, and its doubling time, 53 years. The doubling time for South Korea was 35 years; for 
Taiwan, 36 years; Singapore, 43 years; and Hong Kong, 50 years. The People's Republic of 
China presents a special problem. The size of its population and its growth rate are uncertain. 

Estimates of size range from 800 to 950 million people, the 1975 United Nations figure being 
838 million. The United Nations estimated the growth rate as 1.7 percent (a 55-year doubling 
time), but this is basically an informed guess. 

For the rest of Asia, doubling times tell the familiar story of the LDCs: Philippines, 21 years; 
Pakistan, 22 years; Malaysia, 24 years; Indonesia, 27 years; Afghanistan, 28 years; and India, 29 
years. Among the most rapidly growing countries today arc the Middle-Eastern, mainly Moslem, 
countries of southwestern Asia. The influx of wealth from oil in those lands in the 1970s may be 
effectively suppressing death rates, which until recently were relatively high. Typical 1975 
doubling times were: Kuwait, 10 years; Iran, 23 years; and Saudi Arabia, 24 years. Projections 
for the year 2000 put Asia's population between 3000 and 4500 million and uniformly predict 
that that continent will continue to be the home of more than half of all Homo sapiens. 


TABLE 5-6 

World Population— Three Estimates 


Population (mid- 1975, in millions) 3988 

Average birth rate (per 1 000) 
Average death rate (per 1 000) 
Rate of natural increase (%) 





Environmental Fund 


U.S. Agency 

for International 




28.2 ( 1974) 



2.2 ( 1975) 




Annual increment in population 
(mid- 1970s, in millions) 

Sources: United Nations estimates appeared in United Nations, Concise report; those of the 

U.S. Agency 
United for International 

Nations Environmental Fund Development 

Environmental Fund, Inc., in World population estimates, 1975; and AID estimates in R. T. 
Ravenholt, Gaining ground. 

Even the inadequate and controversial data available are nevertheless more than sufficient for 
our discussion. Whether there actually were only 3.8 billion people or as many as 4.3 billion in 
the world in mid- 1976, or whether the average worldwide growth rate was 1 .5 or 2.2 percent, 
does not significantly affect our conclusions about what that implies for the future of the planet 
and its inhabitants. 


Once there was a young man who proposed a novel pay scheme to a prospective employer. For 
one month's work he was to receive I cent on the first day, 2 cents on the second, 4 on the third, 
and so on. Each day his pay was to double until the end of the month. The employer, a rather 
dull-witted merchant, agreed. The merchant was chagrined, to say the least, when he found that 
the young man's pay for the fifteenth day was more than $160. The merchant went bankrupt long 
before he had to pay the young man $167,733 for the twenty- fifth day. Had he remained in 
business, he would have had to pay his new employee wages of more than $10 million for the 

This is just one of many stories illustrating the astronomical figures that are quickly reached by 
repeated doubling, even from a minute base. Another is about a reward that consists of a single 
grain of rice on the first square of a chessboard, 2 on the second, 4 on the third, and so forth, 
until the board is filled. It turns out that completing the reward would take several thousand 
times the world's annual rice crop. 

Equally horrendous figures may be generated by projecting the growth of the human population 
into the future. Doubling time for that population fluctuated around 35-37 years between 1960 
and 1974. If growth continued at that rate, the world population would exceed a billion billion 
people about 1000 years from now. That would be some 2000 persons per square meter of 
Earth's surface, land and sea! Even more preposterous figures can be generated. In a few 
thousand more years everything in the visible universe would be converted into people, and the 
diameter of the ball of people would be expanding with the speed of light! Such projections 
should convince all but the most obtuse that growth of the human population must stop 

Age Composition 

The discussion of the human population so far has dealt mainly with population sizes and growth 
rates, but of course there is more to demography than that. Populations also have structure: age 
composition and sex ratio, as well as distribution and dispersion (the geographic positions and 
relative spacing of individuals), discussed later in this chapter. These structural factors can 

strongly influence rates of growth and do have profound effects on the social and economic 
conditions under which a population lives. 


MEXICO |t*7Q) 

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llriin;io UK m '19 


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— TS 71 

— 70 74 

— «-fi§ 

— CO M 

— » » 

— 50-5* 

— 44 ■» 

— 40 44 

— 34 » 

— 30- 34 

— » 24 

— 1S 19 

— 10 14 
1— S. 9 

— 04 

— I^^M I 


FIGURE 5-10 

The age structure of a population is profoundly affected by changing fertility. In a country like 

Mexico, with a recent history of high fertility, the age structure is pyramidal. In a country like 

Sweden, with a recent history of low fertility, the structure is quite rectangular up to age 60. The 

U.S. age structure lies between the two extremes. (Adapted from Freedman and Berelson, 1974.) 

Population profiles are a graphic means of showing the age composition of a population—that is, 
the relative numbers of people in different age classes. Look, for instance, at the age 
compositions of the populations of Mexico, the United States, and Sweden in 1970, as shown in 
Figure 5-10 . Because the profiles are based on proportions, they all have the same area, despite 
great differences in the absolute sizes of those populations. This allows you to focus easily on 
their shapes, which of course reflect the different age compositions. Mexico's profile exemplifies 
rapidly growing countries with high birth rates and declining death rates. Most of its people are 
young (48 percent are under age 15). Sweden, however, has had low birth and death rates for 
many decades. It has a much slenderer population profile than Mexico. Only 21 percent of the 
people of Sweden are under age 15. The irregular profile exhibited by the United States reflects 
relatively large fluctuations in the birth rate over the past fifty years, particularly the baby boom 
of the late 1940s and the 1950s. The recent decline in American fertility is apparent in the 
constriction at the pyramid's base—the smaller numbers in the 1—4 and 5—9 age classes. The 
percentage of persons under age 15 was 27 in the mid-1970s, a somewhat higher percentage than 
in Sweden. 

In Mexico— and most LDCs— high birth rates and increasing control over mortality (especially 
infants and children) have greatly inflated the younger age groups in the population since World 

War II. There has not yet been time for individuals born in the period of death control to reach 
the older age classes, whose death rates are higher than those of the younger age classes. In most 
LDCs the greatest declines in death rates among infants and children occurred in the late 1940s, 
and the large numbers of children born in that period began to reach their peak reproductive 
years in the late 1960s. Their children in turn are further inflating the lower tiers of the 
population pyramid. 

Eventually, of course, either population control will reduce birth rates in those countries, or 
famine or other natural checks on population will once again increase mortality in the youngest 
age classes—or, possibly, in all 




T t 


FIGURE 5-11 

A. Age composition of the population of India in 1951. Declining death rates had not yet 

produced the pinched profile characteristic of rapidly growing LDCs like Mexico. (Adapted from 

Thompson and Lewis, 1965.) B. Age composition of the population of India in 1970; the profile 

then resembled a typical LDC. (After Population bulletin, vol. 26, no. 5, Nov 1970. Courtesy of 

the Population Reference Bureau, Inc., Washington, D.C.) 





Deppoder* ages 


Wwhirtg m 

FIGURE 5-12 
Dependency loads in Mauritius (1959), India (1951), Japan (1960), the United States (1960), and 
the United Kingdom (1959). Note the contrast in proportions of economically active people in a 

typical LDC, Mauritius, and a typical DC, the United Kingdom. (Adapted from Population 
bulletin, vol. 18, no. 5, courtesy Population Reference Bureau, Washington, D.C.; and Thompson 

and Lewis, Population problems, 1965.) 

age classes. If birth rates are lowered, eventually there will also be rises in the death rate as the 
populations age and the older age classes with higher age-specific death rates become 
proportionately larger. 

In the absence of both birth control and natural checks, however, death rates in the 
extraordinarily young populations of the LDCs may temporarily fall below those of the DCs. For 
instance, in the early 1970s the death rate in the United Kingdom was 1 1.7 per 1000; in Sweden, 
10.5; in Belgium, 1 1 .2; and in the United States, 9.4. In contrast, the death rate in Costa Rica was 
5.9; in Mexico, 8.6; in Trinidad, 5.9; in Sri Lanka, 6.4; in Singapore, 5.2; and in Hong Kong, 5.5. 

43 The low death rates are a product of the age composition of the populations in those places. 
They do not, for instance, necessarily reflect a better level of medical care or longer life 

One additional profile shape is common: the triangle characteristic of countries that have both 
high birth rates and high death rates. Such profiles must have typified most human populations 
until fairly recently. They lack the extremely broad base of profiles like that of Mexico ( Figure 
5-10 ) and most other LDCs today. India's profile was essentially triangular in 1951 ( Figure 5-11 
). Since then, India's death rate has dropped by about 10 per 1000, and the base of the profile has 
broadened to produce the "pinched triangle" shape of other LDCs. 

One of the most significant features of the age composition of a population is the proportion of 
people who are economically productive to those who are dependent on them. For convenience, 
the segment of the population in the age class 15-64 is chosen as an index of the productive 
portion of a population ( Figure 5-12 ). Figures 5-10 , 5-11 , and 5-12 p rovide a comparison of the 
proportion of dependents in the populations we have been considering. The proportion of 
dependents in LDCs is generally much higher than in the DCs, primarily because such a large 
fraction of the population is under 15 years of age. Thus, the ratio of dependents to the total 
population is higher in the poor countries and lower in the rich countries, although the ratio is 
somewhat misleading because of the greater utilization of child labor in LDCs. This unfortunate 
dependency ratio is an additional heavy burden to the LDCs as they struggle for economic 
development (see Box 5-3). 

The high percentage of people under 15 years of age in LDCs is also indicative of the explosive 
potential for growth of their populations. In most LDCs this percentage is 40 to 45; in a few, as 
high as 50. By contrast, the percentage of persons under age 15 in DCs is usually between 20 and 
30. Thus, LDCs have a much greater proportion of people in their prereproductive years. As 
those young people enter their reproductive years, the childbearing fractions in those populations 
will increase greatly. In turn, their children will further inflate the youngest age groups. These 
masses of young people in the LDCs are the gunpowder of the population explosion. 

Birth, death, and fertility rates. The birth and death rates used thus far, which are expressed in 
births and deaths per 1000 in a population per year, are called by demographers crude birth rates 
and crude death rates. They are called crude because they do not contain information about 
differences in the age composition of populations. They are simply estimates of b and d (Chapter 
4); and the difference between them (b — d = r) is the annual growth rate. Crude birth rates and 
death rates are the most readily available demographic statistics (and thus most widely quoted), 
but although they are often quite useful, comparison of crude rates can be misleading. 

An outstanding example of this was the highly publicized decline in the birth rate of the United 
States during the 1960s, which was widely misinterpreted as heralding the end of the United 
States population explosion. The birth rate in 1968 was 17.4 per 1000, a record low for the 
country-below the previous low of the Depression year 1936. (The trend in birth rates between 
1910 and 1973 is shown in Figure 5-13 .) Between 1959 and 1968 the crude birth rate declined 
about 25 percent. Closer examination of the data, however, shows that the decrease was caused 
only in part by a drop in the number of children born to the average couple. Some of the decrease 

was because a smaller percentage of the population was in the childbearing years. The relevant 
demographic statistic here is the general fertility rate. 

Population Reference Bureau, Inc., 1975 Population data sheet. 
-205- PAGENUMBER206 

BOX 5-3 Mauritius, Children, and the Dependency Load 

Mauritius, one of the Mascarene Islands in the Indian Ocean, has one particular claim to fame—it 
was once the home of the now-extinct dodo, a flightless bird larger than a turkey. But Mauritius 
also presents another case history in population biology—this one concerning Homo sapiens. By 
1969 more than 800,000 people were jammed onto the island— more than 440 per square 
kilometer— and the population had a growth rate of 2 percent a year. The story of Mauritius' 
population growth after World War II is similar to that of other less developed countries, and the 
result by around 1960 was a dependency load of 47 percent. That is, 47 percent of the population 
was either younger than 15 (44 percent) or older than 65 (3 percent). Although the proportion 
under 15 had by 1975 been reduced to about 40 percent (the growth rate had dropped to 1 .7 
percent), the society is still faced with a tremendous burden in the form of vast numbers of 
children who are nonproductive, or relatively unproductive. 

The huge proportion of young people in the population has put a tremendous strain on the 
Mauritius school system. Many primary schools had to go on double shifts during the 1960s, and 
an extreme shortage of teachers developed. In order to staff the schools, teachers had to be put in 
charge of classes before they completed their training. Most of the country's educational effort 
went into primary schools; in 1962 only I out of 7 elementary-school students went on to high 

The education problem in Mauritius is a good example of what demographer Kingsley Davis 
meant when he said that children "are the principal victims of improvident reproduction." Many 
LDCs cannot afford to educate their children adequately— in part, because of a more urgent need 
for using their limited funds for public health and welfare. After the first five years of life, 
children in the LDCs enter the age class that has the lowest mortality rates. Those children 
desperately need education— for their own good and for the future of their societies-but the sorry 
fact is that in the absence of population control, solving health problems in LDCs like Mauritius 
makes solving the education problem extremely difficult. 

High fertility and low income also tend to force children out of school and into the labor pool as 
early as possible. Even the chances for education in the home are reduced when families are 
large and mothers overburdened. Child labor is used at a very high level in less developed 
countries, although the productivity of that labor may be quite low. United Nations statistics, 
which may understate the case, show that 31 percent of males between 10 and 14 years of age 
are economically active in LDCs, in contrast to only 5 percent in DCs. 

It is a brutal irony that children thus must bear the brunt of the population explosion. 
Commenting on the complacency with which many of the rich formerly regarded the plight of 

the poor, Davis wrote, "the old philosophy that [the children's] coming is a just and divine 
punishment for their parents' sexual indulgence, and therefore not to be mitigated by deliberate 
control is one of the cruelest doctrines ever devised by a species noted for its cruel and crazy 

Most of the information here is from Kingsley Davis, The population impact on children in the 
world's agrarian countries. 

The general fertility rate (the number of births per 1000 women 15 to 44 years old) is a more 
refined indicator of birth trends because it compensates for differences in sex ratio (which may 
be the result of wars or migration of workers) and for gross differences in age composition. 
Differences in age composition would be revealed if two populations had identical crude birth 
rates but widely differing fertility rates. This could mean, for instance, that one had a relatively 
small proportion of people 15 to 44 years old and a high fertility rate, whereas the other had a 
relatively high proportion in the 15 to 44 age group and a lower fertility rate. 

As is shown in Figure 5-14 , the fertility rate in the United States was declining along with the 
birth rate during the period between 1959 and 1968. Thus, not only were fewer babies being born 
in proportion to the entire population than during the 1950s, but fewer babies were being born in 
relation to the population of females in 

-206- PAGENUMBER207 

!,"■ ,H*-M 

1910 ih.1i 1W0 'MO mo 1H0 >9TO HIS 

FIGURE 5-13 Birthrates in the United States, 1910-1974. (Adapted from Population profile, 

Population Reference Bureau, March 1967. Courtesy of the Population Reference Bureau, Inc. 

Washington, D.C. Recent data from U.S. Bureau of the Census. 

'i : 


FIGURE 5-14 Fertility rates in the United States, 1910-1974. (Adapted horn Population profile, 
Population Reference Bureau, March 1967. Courtesy of Population Reference Bureau, Inc., 
Washington, D.C. Recent data from U.S. Bureau of the Census.) 
-207- PAGENUMBER208 

BOX 5-4 Stable Population Models 

A stable population is one in which vital rates (fertility and mortality) and age composition 
remain constant. The population itself may be growing or shrinking, either slowly or rapidly, or 
its size may be stationary. The models shown in Table 5-7 represent two stationary and five 
growing populations resembling actual populations that exist today or that may have existed in 
the past. The resemblance, of course, is only general; the models are hypothetical projections of 
demographic factors and trends. But they are helpful for seeing how those factors and trends are 

Model 1, for instance, is thought to be similar to early, preagricultural populations. Recent 
research suggests that both fertility and mortality may have been slightly lower than shown here, 
but in other respects the model probably fits reasonably well. _Early human populations are 
believed to have been close to stationary, as Model 1 is. They grew only very slowly over many 
thousands of years — no doubt, with many short-term fluctuations above and below the no- 
growth level. 

Model 2 represents an agricultural population with high birth rates and mortality somewhat 
lower than primitive levels. If western Europe during the Industrial Revolution had maintained 
constant vital rates at these levels for a lifetime, the population structure would have resembled 
this model. Earlier favorable periods in history, such as the heyday of the Roman Empire and 
Japan before the Meiji Restoration, may have seen life expectancies and vital rates approaching 
those of Model 2. Less developed countries in the twentieth century also had similar vital rates as 
modern death control was introduced, but for them this stage was even more transient than it was 
for industrializing Europe. Demographic stability was clearly never achieved by those LDC 

By the 1970s mortality in most LDCs was much lower, and life expectancies and mortality rates 
resembling those in both Model 3 and Model 4 could be found among them. Life expectancy in 
India, Bangladesh, and several African countries, for example, was around 50 (Model 3). In 
some LDCs, on the other hand, life expectancy was approaching 70, a level typical of developed 
countries. Among those are Singapore, Hong Kong, Taiwan, and several Latin American 
countries. Many of these countries still have high birth rates and are growing very rapidly, as in 
Model 4. 

The last three models are intended to represent developed countries that have experienced a 
demographic transition and have high life expectancies and low mortalities and fertilities. A few 
European countries have had low fertility long enough that their populations resemble Model 5 
fairly closely. A life expectancy of 74.8 years, shown in Model 6 and Model 7, is postulated as 
the highest attainable under optimum conditions with present or foreseeable medical technology. 
Given this life expectancy, replacement fertility is 2.08 children per woman, and both the crude 
birth rate and the crude death rate would be 13.4 once stability was achieved. This is the situation 
in Model 7, a stable, stationary population with low fertility and mortality. 

their childbearing years. Yet, the 1968 fertility rate of about 85, unlike the crude birth rate, was 
still higher than the lows reached during the Depression (when the figure was well below 80) and 
the population was still growing at about 1 percent per year. 

In 1970 the fertility rate began to rise, reflecting a rising proportion of women in their twenties 
(the prime reproductive years). The rise was brief, however; by the end of 1974 the fertility rate 
in the United States had dropped precipitously to 68.4. Moreover, this did not reflect a change in 
age composition but, rather, a significant change in reproductive behavior. For the first time in 
history, fertility in the United States had dropped below replacement level — below a net 
reproductive rate of 1 . (The NRR at the end of 1974 was 0.9.) 

Momentum of Population Growth 

Let us review for a moment some of the basic population dynamics discussed in Chapter 4. If 

D. E. Dumond, The limitation of human populations; and G. B. Kolata , IKung hunter- 

TABLE 5-7 

Models of Stable Populations 

High-fertility models Low-fertility models 


Feature Model 2 Model 3 Model 4 Model 5 Model 6 Model 7 

Children born per woman 6.0- 6.0 6.0 6.0 2.5 2.5 2.08" 

High-fertility models Low-fertility models 

Feature Model Model 2 Model 3 Model 4 Model 5 Model 6 Model 7 

(total fertility) 

Life expectancy (years) - 21.3 - 30.0 50.0 70.0 70.0 74.8 74.8- 


Births per 1000 population 

Deaths per 1000 population 

Rate of natural increase 
per 1000 















0.0 12.1 27.6 36.5 5.2 6.1 0.0 









Doubling time (years) 









All ages 








Ages 0-14 








Ages 15-64 








Ages 65 years and older 








Dependency ratio - 








Median age (years) - 








"That level of fertility that, 

at the given 



exact replacement (NRR = 1) 

For both sexes combined. 

c That level of mortality that, at the given fertility, ensures exact replacement. 
^Numbers aged 0-14 and 65 and over per 100 aged 15-64. 
e Age so determined that half the population is below and half is above that age. 
Source: United Nations, Concise report. 

Certain interesting relationships emerge from a comparison of the models. For instance, the birth 
rate drops steadily between Models 1 and 4, although the fertility per woman remains the same. 
The reason is changing age compositions. Lower mortality results in higher survival rates among 
infants and children and thus increases the proportion of people younger than the childbearing 
ages. In a population with high fertility, indeed, the crude death rate can fall to a much lower 
level than in a low-fertility population because the proportion of old people with high age- 
specific death rates (even with the best medical care) is very small. 

specific vital rates (birth and death) remain constant, the age composition of a population 
eventually becomes stable, a situation in which the proportion of people in each age class does 
not change through time. A population with a stable age composition can be growing, shrinking, 
or constant in size. Box 5-4 shows a series of stable population models in which the reciprocal 
influences of vital rates and age compositions can be seen. Five of the models represent growing 
populations; two represent populations that are constant in size. 

When a population is constant in size (the crude birth rate equals the crude death rate), 
demographers refer to it as stationary. Colloquially, one says that zero population growth (ZPG) 
has been achieved. Replacement reproduction means that each married couple, on the average, is 
having just the number of children that will lead to the parents' replacement in the next 
generation: NRR = 1 . (For a full explanation of NRR, refer to Chapter 4.) Where death rates are 
at typical DC levels, this is about 2.1 1 children per woman. The extra 0.1 1 child per 


woman compensates for prereproductive mortality, nonmarriage, and infertility in that 

The NRR (R o) of a human population is the ratio of the number of women in one generation to 
that in the next. It is calculated by applying the age-specific birth and death rates of the 
population at a given time to a hypothetical group of 1000 newborn female babies, determining 
how many live female babies those females would themselves produce, and dividing that number 
by 1000. 

If the average completed family size of a population with typical DC death rates is three 
children, the NRR will be about 1 .3. An NRR of 1 .3 means that, barring changes in birth and 
death rates and assuming a stable age composition, the population will grow 30 percent per 
generation (a generation is usually about 25 to 30 years). As long as the NRR is more than 1, 
such a population will continue to grow. An NRR of 1 (fertility at replacement level) indicates 
either a stationary population or one that will become stationary after time has allowed the age 
composition to stabilize. If the NRR drops below 1 and stays there, the population will shrink 
sooner or later (how soon depends on the initial age composition). 

Note that the NRR describes what the relative sizes of consecutive generations will be if the age- 
specific vital rates (death and fertility) remain constant at the values they had when the NRR was 
calculated. Even if those rates (and thus the NRR) do not change, the age composition of the 
population must be known in order for population projections to be made, since Homo sapiens 
has overlapping generations. 

Population momentum in the United States. The drop in American fertility to below 
replacement level between 1972 and 1975 was popularly interpreted to mean that zero 
population growth (ZPG) had been achieved in the United States. But growth certainly had not 
stopped (natural increase in 1975 was 5.8 per 1000; immigration brought the growth rate to about 
8 per 1000). Nor will it stop until the crude birth rate is balanced by the crude death rate (and 
immigration is balanced by emigration). Because of the age composition of the population, if 
replacement reproduction (NRR =1) were exactly maintained for about seventy years (one 
lifetime), then natural increase would cease. Population growth has momentum — growth does 
not stop instantaneously when each couple, on the average, just replaces itself. 

If low fertility were maintained long enough, the age composition would change in response, and 
the median age of the United States population would gradually rise from about 28 (in 1975) to 
37. The rising proportion of older people in the population would result in higher death rates; and 

because there would be proportionally fewer young people reproducing, the crude birth rate 
would decline slowly. Eventually the birth rate and the death rate (in 1975 about 14.8 and 9.0, 
respectively) 43a would converge at about 13 per 1000. Past and projected changes in the age 
composition of the United States population from 1900, when the population was growing 
rapidly, to an ultimate stationary population (assuming continued replacement fertility) are 
presented graphically in Figure 5-15 . 

If the United States population maintained the below-replacement-level fertility of 1973 to 1976 
(NRR = 0.9), growth would end by the year 2025 with a peak population of about 252 million 
(even if present legal immigration rates continued). ^_After that the population would gradually 
decline in size. 

It is impossible to say at this point, however, whether the low fertility figures for the early 1970s 
signal the beginning of a sustained trend or whether they are merely examples of the short-term 
ups and downs that for the past few decades have confounded the demographers and economists 
who have tried to explain these things. Many factors can influence birth rates in addition to the 
number of women in their childbearing years. Severe economic conditions, epidemics, and wars 
may cause declines in birth rates. For instance, the shipment of young men overseas during 
World War I and the great influenza epidemic of 1918 together led to a drop in the United States 
birth rate from 28.2 in 1918 to 26.1 in 

3a U.S. Bureau of the Census, "Annual summary for the United States, 1975", Monthly vital 
statistics report, vol. 24, no. 13, June 30, 1976. 

United States Bureau of the Census, Current population reports, population estimates and 
projections, Washington, D.C., series P-25, no. 541, February. 1975. 
-210- PAGENUMBER211 


US : 1M0 

U.S.: 1970 


l I I II I I I 


U.S.: 2000 


80 84 — 

75- 79 — 

70 74 — 

65-69 — 

60 64 — 

65 59 _ 

50 54 — 

45 49 — 

(3 40 I ; — 

35 39 — 
30-34 — 
25 29 — 
20-24 — 
15 T9 — 
10- 14 — 
5-9 — 
0-4 — 

6 543210 123456 






80-84 _ 

J I 

75- 79- 


70- 74 — 




60-64 — 









45-49 — 



040-44 — 
35- 39 — 





30 34 — 



25- 29 — 


20- 24 — 


IS- 19 — 


10 14 — 


5-9 — 


0-4 — < 


i i i i i i 

6 5 4 3 2 1 i 


3 12 3 4 5 6 


FIGURE 5-15 A. The U.S. population in 1900 had the age composition represented by this 

pyramid. Its triangular shape, strikingly similar to that of India in 1951 ( Figure 5-11 ), is 

characteristic of a fast-growing population with high birth and death rates, where the average life 

expectancy is less than 60 years. A third of Americans were then under 15 years of age. B. The 

U.S. population of 1970 gave rise to a pyramid whose sides were pinched in because of the low 

birth rates that prevailed during the years of the Great Depression. The bulge centered on the 10- 

to- 14-year-old age group is a consequence of the postwar baby boom. C. The U.S. population of 

the year 2000 will form this age pyramid if fertility stabilizes at replacement levels from now 

until the end of the century. Five-to- 19-year-olds of 1970, who will then be 30 years older, will 

have produced a second bulge of 5-to- 19-year-olds. D. An ultimate stationary population, if it is 

achieved in the United States during the next century, will have the age composition shown here. 

A third of the population will be less than 25 years of age, a third will be between 25 and 50, and 

another third will be over 50. (Adapted from Westoff, 1974.) 

1919. Similarly, low birth rates in the United States and Europe in the 1930s have been attributed 
— perhaps somewhat erroneously — to the economic hardships of the Depression. ^_And 
improvements in economic conditions and the return of servicemen after World War II led the 
birth rate in the United States to jump from 20.4 in 1945 to 26.6 in 1947. 

The present low fertility rate in the United States may result from a combination of social forces 
now at work in our society: rising public awareness of the consequences of overpopulation, the 
growth of the women's liberation movement, the extension of family-planning services to low- 
income groups, and the legalization of abortion. (See Chapter 13 for more discussion of these 
social influences.) If these are the most important influences on reproduction in the United 
States, consistently low fertility may continue into the future. If, on the other hand, fertility is 
determined by economic factors such as the unemployment rate among prospective fathers, then, 
as economic conditions improve, fertility may rise again and continue to fluctuate over the long 
term thereafter. 

From statistics on births over the past four decades, it is known that the number of women in the 
1 5 to 44 age class in the United States has been increasing and will continue to increase 
throughout the 1970s (by 12 percent between 1974 and 1980). Moreover, the age 20 to 29 
subgroup, whose members bear most of the children, is increasing even faster. It will grow by 
about 14 percent between 1974 and 1980 and will not reach a peak until after 1980. 46 Therefore, 
unless fertility in the age 20 to 29 cohort falls considerably below its 1974 level, both the crude 
birth rate and the general fertility rate in the United Statescaw be expected to rise during the late 
1970s and early 1980s. 

Some evidence is emerging that the extremely low fertility recorded in the early 1970s was in 
part a result of later marriage and postponed childbearing among young women, combined with 
a reduction of fertility among women over 30, most of whom had married and started their 
families at relatively younger ages. Although the marriage rate has been dropping and rates of 
divorce and separation have risen steadily, by 1975 there was a large pool of young married 
women — one-third of all evermarried women younger than 30 — who had not borne children (in 
1960 the proportion was about 20 percent). Demographers J. Sklar and B. Berkov speculated that 
that age group was likely to start its postponed childbearing in the late 1970s, causing a 
significant rise in the national birth rate by 1977. — 

The U.S. National Fertility Survey conducted by the Bureau of the Census in 1974 indicated that 
84 percent of all currently childless wives under 30 years old anticipated having one or more 
children, but among those over 25 the percentage dropped to 73. ^_Since other surveys have 
shown an aversion among Americans to having one-child families, ^Jhe further assumption is 
made that most of those women eventually will bear at least two children. 

Sklar and Berkov may be right. A burst of postponed births among married women around the 
age of 30 could cause a rise in the birth rate and in the general fertility rate between 1975 and 
1985. But by late 1976 there was no evidence of such a rise (fertility, indeed, declined further in 
early 1976). And what really counts in the long run is completed family size (the average number 
of children per woman, sometimes called total fertility). The 1974 Fertility Survey indicated a 
strong trend toward the two-child family, especially among younger women ( Table 5-8 ). Wives 
aged 25 to 29 in 1974 expected an average total of about 2.3 children, and those 18 to 24 
expected only 2.17 (approximately replacement level). If their expectations are realized and if 
the trend continues with younger cohorts, the population is on its way to ZPG, regardless of 
temporary fluctuations in the birth rate caused by differences in age composition and the timing 
of births. 

Demographer Thomas Frejka, using 1965 as a base year, showed what could happen to the 
United States 

See, for instance, Richard A. Easterlin, Population, labor force, and long swings in economic 
growth; A. Sweezy, The economic explanation of fertility changes in the U.S. Easterlin 
supports the conventional view, whereas Sweezy argues that the low birth rates of the 
Depression years were mainly a continuation of a long-established trend. 

46 USNCHS, Vital statistics, vol. 23, no. 12 ( February 28, 1975). 
J. Sklar and B. Berkov, The American birth rate: Evidences of a coming rise. 
United States Bureau of the Census, Prospects for American fertility: June 1974, Population 
characteristics, current population reports, Washington, D.C., series P-20, no. 269 
(September 1974). 

49 Judith Blake, "Can we believe recent data on birth expectations in the United States?", 
Demography, vol. 1 1 ( 1974), no. 25, pp. 25-44. 
-212- PAGENUMBER213 

TABLE 5-8 

Births to Date and Lifetime Births Expected by Married Women 18-39 Years Old, 

Surveyed 1967-1974 

Total Age class at date of survey (years) 

18-39 18-24 25-29 30-34 35-39 


1974 2550 2165 2335 2724 3091 

1971 2779 2375 2619 2989 3257 

1967 3118 2852 3037 3288 3300 


Age class at date of survey (years) 
18-24 25-29 30-34 35-39 

1691 2539 3063 

1949 2802 3210 

2312 3050 3214 

Source: United States Bureau of the Census, "Prospects for American fertility: June 1974", 

Current population 

reports, population chracteristics , series P-20, no. 269 ( September 1974). 

'H.I r 






















FIGURE 5-16 

Projection of changes in the crude birth rate that would be necessary if the total population of the 

United States were to remain constant during the period from 1965 to 2365. (Adapted from 

Frejka, 1968.) 

population under a variety of assumptions. ^_For instance, instant ZPG (a stationary population) 
could be achieved only by reducing the NRR to slightly below 0.6, with an average of about 1 .2 
children per family, between 1965 and 1985. Thus, to bring the crude birth and death rates 
immediately into balance (so the growth rate was 0), the average completed family size would 
have to drop far below the replacement level that would eventually lead to such a balance. After 
that, in order to hold the population size constant, the crude birth rate and NRR would have to 
oscillate wildly above and below the eventual equilibrium values for several centuries (see 
Figure 5-16 ). The age composition would correspondingly change violently, undoubtedly 
having a variety of serious social consequences. 

The problems caused by great fluctuations in the birth rate and irregularities in the age 
composition could be avoided by maintaining the 1975 level of fertility (slightly below 
replacement). This would produce further growth, but at a slackening rate. Disregarding 
immigration, growth would end in about fifty years with a peak population of about 252 million, 
and then there would be a slow decline. Accepting some further growth followed by a period of 
negative population growth 


Reflections on the demographic conditions needed to establish a U.S. stationary population 


19€fi TO 








FIGURE 5-17 Projections of the course of population growth in the United States if the NRR 

reached 1 in various years. The total population size would be slightly less than twice the size of 

the female population, since there are slightly fewer men than women. (Adapted from Frejka, 


(NPG, r negative rather than attempting to hold the population precisely at ZPG, would seem to 
be much less disruptive. And, as we discuss throughout this book, there are powerful arguments 
for reducing the size of the United States population well below its present level (to say nothing 
of any projected future peak). 

Frejka also described what would happen if the NRR declined from the 1965 level of about 1.3 
to 1 in a series of years starting with 1975 (assuming there was no immigration). The projections 
in Figure 5-17 clearly indicate that substantial population growth would occur after a pattern of 
replacement reproduction was established, no matter when that might be. For instance, if an 
NRR of 1 had been reached in 1985 and maintained exactly, the population would not have 
stopped growing until 2055, and the ultimate population would have been around 300 million 
(slightly less than twice the female population size shown in the figure). If the NRR of 1 reached 
by 1973 were more-or-less precisely maintained, the population would still grow to at least 280 
million, not stopping before the year 2035. Of course, the ultimate number of people would be 

much higher if the NRR rose again above 1 for any significant length of time, and it would be 
lower if it remained below 1 . 

Reflecting the dropping fertility rate since the late 1950s and a rising public consciousness of 

-214- PAGENUMBER215 


3S0 - 

3W - 

1840 1W4 IBM ISH& 1MO 1M& 1»7D lift ittQ 1| 

'W l«i JOB 2004 »1« Kit K» W?F, 

FIGURE 5-18 Estimates and projections of the population of the United States, 1940-2025. ( 

U.S. Bureau of the Census.) 

growth, the Bureau of the Census has periodically issued revised projections of U.S. population 
growth, each a little lower than the last. Since 1970, the bureau has given at least one alternative 
projection each time that was based on the attainment of replacement fertility in the 1970s, with 
slight fluctuations around that point between 1970 and 2000 to compensate for age composition 
differences. Projections issued in 1975 51 ranged between 245 million and 287 million for the 
year 2000 ( Figure 5-18 ). All three projections in 1975 assumed continued net immigration 
(legal) at present levels (400,000 per year) and slight reductions in mortality. Fertility 
assumptions, however, differed: Series I assumed a completed fertility of 2.7 children per woman 
(as in the early 1960s), Series II assumed a completed fertility of 2.1 (approximately 
replacement), Series III assumed a completed fertility of 1.7. Series II (if immigration were 
ended) would ultimately result in a stationary population around the middle of the twenty- first 
century. Series III would result in a peak population of about 252 million around the year 2020, 
followed by negative population growth — a gradual decline. 

A fertility picture resembling that of the United States prevails in other developed countries. 
Figure 5-19 shows fertility trends in many DCs since World War II. Many demographers now 
believe that what is happening is a consummation of the demographic transition, which will 


ultimately end in ZPG for those countries. The postwar baby boom, viewed in this perspective, 

seems to be a temporary reversal of a long-term trend. Indeed, as the figure shows, it was either a 

relatively minor zig or else was virtually nonexistent in all DCs except Great Britain's former 
colonies ~ the United States, Canada, Australia, and New Zealand. 

51 Projections of the population of the United States: 1975 to 2050, Population estimates and 
projections, current population reports, series P-25, no. 601 (October 1975). For a discussion 
of the recent series of census bureau projections, see Leon F. Bouvier, U.S. population in 
2000: Zero growth or not? 
C. F. Westoff, The populations of the developed countries . 





— Canada 

^^- y "\ ^New Zealand 






? 5 

1 L 

L Australia 


U.S.^ ^*>\ 












i i 

1 1 1 ! 

1945 1950 

1955 I960 

1965 1970 1973 

1945 1950 1955 1960 1965 1970 1973 

FIGURE 5-19 Fertility since 1945. Total fertility refers to average completed family size. A. 

Since 1945 the United States has averaged 1 .9-3.8 children per family. The postwar baby boom 

was most pronounced among overseas English-speaking populations (top). Sharp declines began 

in the 1960s. The postwar surge in fertility in Western Europe (bottom) was brief. Fertility fell 

sharply, climbed again slowly, and since the early 1960s has been declining. Fertility in Ireland 

is quite different, reflecting its unusual demographic past. B. Slow declines in fertility seem to be 

taking place in Portugal and Italy (top). There is no clear trend in Spain or Greece, but data are 

limited. In the Communist countries of Eastern Europe (bottom), fertility has generally been 

falling, except for a brief, sharp rise in Rumania 



1945 1950 

45 r 


1955 1960 



1970 T973 

3 5 

£ 30 


£ 25 

£ 20 

2 15 





East Germany 

West Germany 




1945 1950 1955 1960 1965 1970 1973 

when the abortion law was tightened in 1967. C. Scandinavian countries, except for Sweden, 

showed only a brief postwar surge in fertility. The decline since then has been sharpest in 

Finland. Countries of Central Europe have followed a fertility pattern similar to that of Western 

Europe. West German fertility is now the lowest among all developed countries, and the country 

has reached zero population growth. D. Two newly developed countries, Japan and Israel, show 

markedly different fertility patterns. The decrease in Japanese fertility followed the adoption of a 

liberal abortion law. The curve for Israel applies only to the Jewish population. Fertility of the 

Arabs in Israel is currently more than twice as high. (Adapted from Westoff, 1974.) 


The total fertility rate is the average number of children each woman would bear during her 
lifetime if age-specific fertility remained constant, and a total fertility rate of 2.1 is roughly equal 
to an NRR of 1 where typical DC death rates prevail. As the figure shows, many European 
countries have reached replacement fertility, and some have dropped even lower. A few 
countries with extremely low fertility or relatively high rates of emigration had achieved 
negative population growth (NPG) by 1975. Among those were East Germany, West Germany, 
Luxembourg, and the United Kingdom. — 

In the most highly industrialized countries of Europe, fertility has been at least moderately low 
for nearly two generations. Relatively little momentum is therefore built into their age 
compositions. Eastern and southern European countries, the USSR, and especially Japan, on the 
other hand, have comparatively short histories of low fertility. As in the United States, 
considerable potential for population growth still exists in those countries. 

Population momentum in LDCs. The situation in rapidly-growing less developed countries is 
radically different, however. The reason for the enormous growth potential inherent in the age 
composition of most LDCs can be seen by considering their schedules of age-specific vital rates. 
In those countries, roughly 40 to 50 percent of the populations are less than 15 years old. Those 
populations of young people will soon be moving into age classes with high age-specific fertility 
rates, but it will be some fifty years more before they are subject to the high death rates 
associated with old age. Fifty years is about two generations, which means that those youngsters 
will have children and grandchildren before they swell the upper part of the age pyramid and 
begin to make heavy contributions to the crude death rate. Therefore, even if the age-specific 
fertility rates in a population dropped precipitously to a net reproductive rate (NRR; R ) of 1, 
ZPG could not be reached for more than fifty years thereafter. You will recall from Chapter 4 
that it takes roughly one life expectancy after NRR (R ,) reaches I before a stable age 
distribution is reached and growth ends (r equals 0). Hence, assuming there is no rise in the age- 
specific death rates, there must be a long braking time before even very successful birth-control 
programs can halt growth in those nations. The momentum inherent in their age compositions 
means that population sizes will expand far beyond the level at which the "brakes" are 
successfully applied. 

Demographer Nathan Keyfitz has calculated the magnitude of that momentum. ^_He 
demonstrated what would happen if a birth-control miracle were to occur and the average 
number of children born to each woman in a typical LDC (currently, the average is about six) 
dropped immediately to the replacement level (perhaps 2.5 children per woman, with present-day 
LDC death rates). 5S If such replacement reproduction were achieved overnight, a typical LDC 
would nonetheless continue to grow until it was about 1.6 times its present size. 

Should the fertility rate of an LDC drop to replacement level gradually over the next thirty years, 
however, the final population would be some 2.5 times the present size. These Keyfitz numbers 
show why it is difficult for many scientists to see how population growth can be brought to a halt 
by birth-control measures before stresses on social systems or ecosystems bring growth to a halt 
by raising death rates. Moreover, a drop to replacement level even in thirty years is extremely 
unlikely to occur in most LDCs. The most optimistic U.N. demographic projections do not 
foresee such a pattern of fertility decline; but even such a relatively happy outcome would 

commit India, for instance, to an ultimate population of some 1.6 billion people if the Indian 
NRR reached I around the year 2005. 

Tomas Frejka has calculated projected paths toward ZPG in detail for the major regions and 
many individual countries of the world, assuming attainment of replacement fertility at different 
times and some decline in death rates from improvements in public health. 56 Figure 5-20 
contrasts the amount of momentum built into the populations of developed countries with those 
of less developed countries. 


United Nations Concise report. The report on the United Kingdom is from International 

Journal of environmental studies, vol. 6 ( 1974), p. 230. 
54 0n the momentum of population growth. 
55 United Nations, Concise report. 

The future of population growth: Alternative paths to equilibrium. For less detailed 

summaries, see Population Reference Bureau, Inc., Population bulletin, vol. 29, no. 5 ( 1974); 

or T. Frejka, The prospects for a stationary world population. 

500 i— 
450 - 
400 |— 





NRR = 1.0 

NRR greater than 1 .0 

Immediate path 

Slow path 

Rapid path 






1970 1990 2010 2030 2050 2070 2090 2110 2130 2150 




450 - 


I 300 
| 250 

O 200 




=» NRR = 1.0 

5S&SS NRR greater than 1.0 

Slow path 

1970 1990 2010 2030 

2050 2070 

2090 2110 2130 2150 

A. Potential population growth in the developed regions by selected paths, 1970-2150. B. 
Potential population growth of the less developed regions by selected paths, 1970-2150. 

(Adapted from Frejka, 1973.) 
TABLE 5-9 
Three Paths to ZPG 



[■ tf i. .■..■! i n 




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■acnnfflu if 






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art te In odhrvtd 

7lm n* 


Tfi* fifkuntr/t una 
art nrcmary 

ImrruJijtt falh 













Rapid path 













Sk*w p*dl 





































1 T 

1990- 19« 


J -J 














l (i 



H 'II 






Tt 40 a 

it in ti ts ia 

I I 

1^ i r j 


M !>d 15 !J0 ?S 


FIGURE 5-21 Age pyramids for India on the slow path to ZPG, 1970-2075. As a population 

moves toward the nongrowing state, a characteristic age structure emerges. Since the numbers of 

births and deaths change very little from year to year, all age groups are approximately the same 

size, although the older groups tend to be smaller because of their higher mortality. (Adapted 

from Frejka, 1974.) 




1 I 


- (TO 

4 300 

- H» 



JHMMU [Mill 

i im i i nn i i im i » 


Rapti pain 

•"-"negate pern 
J I I 












Rajjtd path 

Immediate pah 



-| Wo 


- KB 


! 1SI i I III! 

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Si S 



Frejka's immediate path to ZPG assumes an NRR equal to I starting in the early 1970s. Such a 
course apparently is now being followed more or less closely in many DCs, including the United 
States, but is impossible for LDCs, most of which would have to cut their birth rates by more 
than half overnight to reach it. His rapid path assumes attainment of NRR = 1 by the year 2005. 
This is not impossible for most LDCs, though it is highly unlikely for many. Under this scenario, 
the world's population would be nearly 6 billion in the year 2000 and would continue to grow 
during most of the twenty- first century to at least 8 billion. Frejka's slow path assumes NRR = 1 
being reached by about 2045. The slow path would result in a world population of nearly 7 
billion by 2000, and an ultimate population of 1 5 billion, reached sometime in the twenty-second 
century. Table 5-9 indicates the magnitude of demographic change each path would entail. 

A change in fertility obviously would substantially alter the age compositions of now rapidly 
growing LDCs. The consequences of the slow path for India's population profile are shown in 
Figure 5-21 . Even more radical change would occur — and more quickly — if the rapid path were 

How the more plausible paths would affect population growth in various individual nations is 
shown in Figure 5-22 . (The actual projected population figures for those and other countries 
appear in Appendix 1 .) Even by the immediate path, the United States population is projected to 
reach about 280 million, a 36 percent increase after 1970. Contrasting this with the United States 
Census Bureau projection from the subreplacement fertility rate that prevailed from 1973 to 1976 

of a 252-million peak in 2024, it becomes clear what a large difference a very small difference 

in the fertility rate can make over time. Countries with very high birth rates in 

Stow path 

Rapid path 

Immediate pam 

fl r :fi 

FIGURE 5-23 

Population projections are based on three assumptions about when a net reproduction rate of I 

might be achieved and maintained. Reading from the bottom, the dates are 1970-1975, 2000- 

2005, and 2040-2045. With an index of 100 for the 1970 population, the indexes under the three 

projections would rise by the year 2050 to 153, 224, 357, and by the year 2150 to 156, 230, and 

426, respectively. (Adapted from Frejka, 1973.) 

1970, such as India, Mexico, Nigeria, and even Taiwan (whose birth rate declined significantly 
during the 1960s), can look forward to multiplying their populations five- to tenfold if they 
follow the slow path. 

Figure 5-23 shows the results of Frejka's projections 

FIGURE 5-22 (left) Potential population growth of selected countries by various paths, 1970- 
2150. The index of growth is the percentage of the 1970 population size. (Adapted from Frejka, 



United States Bureau of the Census, Current population reports, series P-25, no. 541 

(February 1975). 

EAST ASIA 1. Mainland region 2. Japan 3. Other East Asia SOUTH ASIA 4. Middle South 

Asia 5. South-East Asia 6. South-West Asia EUROPE 7. Western Europe 8. Southern Europe 9. 

Eastern Europe 10. Northern Europe SOVIET UNION 11. Soviet Union AFRICA 12. Western 

Africa 13. Eastern Africa 14. Middle Africa 15. Northern Africa 16. Southern Africa 

NORTHERN AMERICA 17. Northern America LATIN AMERICA 18. Tropical South America 

19. Middle America 20. Temperate South America 21. Caribbean OCEANIA 22. Australia and 

New Zealand 23. Melanesia 24. Polynesia and Micronesia 

FIGURE 5-24 Regions for which U.N. Demographic projections are made, showing rates of 

population growth during 1970-1975. (Data from Concise report on the worm population 

situation in 1970-1975 and its long-range implications, United Nations, 1974.) 

on the total world population. It must be emphasized that all of these projections assume an end 
to population growth through birth control; they do not consider unpredictable discontinuities 
such as wars and mass famines. Continued growth at high rates like those of the past few decades 
would lead to preposterously huge populations in a surprisingly short time. Even the essentially 
impossible immediate path to ZPG (which would require almost halving the birth rate worldwide 
instantly) commits Earth to a population of nearly 6 billion human beings by 2050. On the more 
reasonable (but by no means easy) rapid path, the population would soar past 8 billion. The built- 
in momentum of population growth thus virtually guarantees that the human population cannot 
be stabilized by means of birth limitation at less than 8 billion people. 

United Nations demographic projections. We have examined what is demographically possible 
in the way of ending population growth by means of birth control, given existing age 
compositions and mortalities. Now let us consider some likelier future paths of population 
change. Of primary interest and significance to us are predictions of population sizes during the 
next century. Such projections have been made by many governmental and private organizations. 


R. T. Raven-holt 

United States Bureau of the Census, The two-child family and population growth: An 
international view. 

•=3"iS» e»o 



holt of the Agency for International Development (AID) has made perhaps the most optimistic 


forecasts (not really detailed projections) we have seen. He claims that family-planning 

programs can reduce the average world birth rate below 20 per 1000 and the growth rate to less 
than I percent by 1985. He postulates a world population of only 5.5 billion by 2000. 

Perhaps the most useful available population projections are those made periodically by the 
United Nations, most recently in 1974 ( Figure 5-24 ) . 60 They are not simple extrapolations of 
past trends or of present rates. Instead, the U.N. projections are computed on the basis of 
individual components; that is, individual forecasts are made of trends in age-specific fertility, 
death rates, migration, and so forth. The forecasts are based on the best available demographic 
data for nations or regions of the world, and the scope of future variation in those rates is 
estimated on the basis of past trends in developed and less developed areas. Possible major 
disasters, such as thermonuclear war or massive famines, are not considered, since they cannot 
be predicted. 

All of the data are integrated to produce medium, low, and high projections, the latter two of 
which the demographers hope will bracket the actual figures. The accuracy of the projections 
depends, of course, on how much the realized rates differ from the predicted rates. 

Gaining ground on the population front. 
60 United Nations, Concise Report. 

TABLE 5-10 

Population of the World and Major Areas in 1975 and in 2075, According to High, 

Medium, and Low U.N. Projection Variants (in millions) 




U.S. and Canada 



East Asia 

Latin America 


South Asia 

Source: United Nations, Concise report. 

2075, according to 



high and 

































































■ Uwllr LDCt 

"^ i^ S££ ^ ^ 

1950 ' » ■' i" 

FIGURE 5-25 Projected growth of the world population, based on U.N. 1963 constant fertility 
projection. (Adapted from Population bulletin, vol. 21, no. 4. Courtesy of the Population 

Reference Bureau, Inc., Washington, D.C.) 

Table 5-10 compares the projected low, medium, and high populations for 2075 for the world 
and for major regions. 

The United Nations formerly made another projection, called the constant fertility, no migration 
projection, which was based on the simpler assumptions that current fertility and the recent 
downward trend in mortality would continue and that there would be no migration between 
geographical areas ( Figure 5-25 ). Detailed presentation of this projection seems to have been 
abandoned, however, as forecasts beyond the year 2000 produced clearly unsustainable numbers 
— total world populations of 7.2 billion in 2000, more than 14 billion in 2025, 33 billion in 2050, 
and 80 billion in 2075!- 

In the past, population projections and forecasts have erred fairly consistently on the low side. 
For instance, in 1948 Time magazine cited the opinions of unnamed experts that a prediction (by 
the Food and Agriculture Organization of the United Nations) of a world population of 2.25 


billion in 1960 was probably too high. (The actual population in 1960 was about 3 billion.) 

In 1949 economist Colin Clark predicted a world population in 1990 of 3.5 billion, and in 1950 
demographer Frank Notestein predicted that by the year 2000 there would be 3.3 billion people 
alive. Both numbers were exceeded well before 1970. In 1957 United Nations demographers 

offered the following population projections for 1970: low, 3.35 billion; medium, 3.48 billion; 
and high, 3.5 billion, The actual population passed the high projection for 1970 sometime near 
the end of 1968. 

In the Depression years of the 1930s it was common for demographers in Europe and the United 
States to show great concern over the possibility of population declines. Their apprehensions 
were based on projections of trends in both birth rate and death rate. But declines in birth rates 
during the Depression were more than compensated for by the baby boom of the 1940s and 
1950s. Moreover, no one foresaw the unprecedented effect of death control exported from DCs 
to LDCs. How widely projected estimates can vary is illustrated in Table 5-11 ., which shows 
United Nations estimates made 

TABLE 5-11 Projections of World Population in 1980 (in millions) as Made by the United 
Nations at Several Points of Time, from 1951 to 1968 

Low Medium High 

Made in variant variant variant 

1951 2976 - 3636 

1954 3295 -- 3990 

1957 3850 4220 4280 

1963 4147 4339 4550 

1968 -- 4457 

Source: Courtesy of Nathan Keyfitz. 

in different years for the world population in 1980. Not surprisingly, the latest estimates will 
probably be closest to the actual figure. 

In 1974, reversing past experience, the United Nations revised downward its estimates of current 
population sizes and its projections for the future for most areas. The changes were based on new 
census results; on the birth-rate declines in Europe, North America, and Oceania; on lower-than- 
expected census results from India; and improved (though still inadequate) data on China. 
Population estimates for Latin America, Africa, and the Soviet Union were revised upward. The 
projections for the medium variant to 2075 appear in Figure 5-26 and in Table 5-12 . Also, for the 
first time the United Nations took its projections to ZPG, on the assumption that a "complete" 
demographic transition in that direction will ultimately occur in all developing countries as well 
as in developed ones. 

Whether the lowered estimates of the 1970 populations are correct is a matter of some 
controversy, at least with regard to the less developed areas. China appears to have made 
substantial progress in lowering fertility (see Chapter 13), but actual data are scanty. The lower 
census figures from India may have been due to undercounting, to a higher-than-expected death 
rate, to reduced fertility, or to a combination of all three. The United Nations postulates a trend in 
India to later marriage, which would have reduced fertility. If the census was incomplete, of 
course, then India's size and rate of growth assume even 


""November 8,1948. 

more alarming proportions. 63 The indicated decline in fertility, should it prove to be real, is 
heartening. But, as both Frejka and the United Nations demographers have clearly demonstrated, 
the momentum of population growth assures that the present population will at least double or 
triple before growth can be ended by reducing fertility. This assumes that the resource base and 
the economic and social fabric that support the human population can somehow be sustained 
under such a strain. On this, even the sober, usually optimistic United Nations appears to have 

It remains justifiable to contemplate also the possibility of severe reversals in the event of major 
breakdowns in international or national organization. There 


FIGURE 5-26 Projected population growth in major areas of the world, 1925-2075, according to 

the U.N. medium variant of long-range projections, charted on a logarithmic scale. (From 

Concise report on the world population situation 1970-1975 and its long-range implications, 

United Nations, 1974.) 
TABLE 5-12 Population of the World and Major Areas, at 25-Year Intervals, 1925-2075 
(medium variant, in millions) 

Area 1925 1950 1975 2000 2025 2050 2075 

WORLD TOTAL 1,960 2,505 3,988 6,406 9,065 11,163 12,210 

NORTHERN GROUP (MOSTLY DCs) 1,203 1,411 1,971 2,530 2,930 3,084 3,107 

U.S. and Canada 125 166 237 296 332 339 340 

Europe 339 392 474 540 580 592 592 

USSR 168 180 255 321 368 393 400 

East Asia 571 673 1,005 1,373 1,650 1,760 1,775 

SOUTHERN GROUP (MOSTLY LDCs) 757 1,094 2,017 3,876 6,135 8,079 9,103 

Latin America 98 164 326 625 961 1,202 1,297 

Africa 153 219 402 834 1,479 2,112 2,522 

South Asia 497 698 1,268 2,384 3,651 4,715 5,232 

Oceania 9 13 21 33 44 50 52 

Source: United Nations, Concise report. 

63 A. Adlakha and D. Kirk, Vital rates in India 1961-1971 estimated from 1971 census data. 
This study accepts the total count and estimates that mortality was higher than anticipated and 
that there was a small reduction in fertility. There is evidence of a trend to later marriage, but 
it may be compensated by there being fewer widows in the reproductive ages. 
-226- PAGENUMBER227 

is no denying that catastrophic events can happen despite all safeguards to avert them .... 
Certainly, international efforts will continue to be directed at the preservation of peace, the 
assertion of human rights, the stimulation of economic and social improvements, the protection 
of the environment and relief action in disaster-stricken areas. Failure in these undertakings 
would have to be immense if the consequences were to have a sizable impact on the high rates of 
population growth which have to be foreseen. It remains imperative to create the conditions 
permitting a healthy life for between 10,000 and 15,000 million human beings which will 
probably inhabit this earth in the coming century. — 

To be complacent about population growth, therefore, one must not only be unworried by the 
condition of today's 4 billion people; one must also be confident that Earth can support the 
population of no less than 8 billion human beings to which humanity is already inescapably 
committed — and (more likely) the 10 to 15 billion the United Nations expects! 

Since prediction is a favorite pastime of almost everyone concerned with population, we will 
offer our own. We believe that United Nations projections for the year 2000 (with the possible 
exception of the low forecast) are too high. This is not, however, because we share their 
optimism about the future impact of family-planning programs on birth rates. Instead, for reasons 
we explain later, we expect that increases in death rates will either slow or terminate the 
population explosion, unless efforts to avoid such a tragic eventuality are mounted immediately. 


Human beings are not uniformly distributed over the face of Earth. Moreover, their distribution 
has continuously changed throughout history, with migration and varying rates of population 
growth. Figure 5-27 shows the pattern of population density that prevailed in 1972. (Population 
density is the number of individuals per unit area.) For human populations, this figure is 
normally expressed as number of people per square kilometer or per square mile. 

Table 5-13 shows the population density of the world's major areas in 1960 and 1975. Note that, 
although most people think of the developed world as being more densely populated, it is not 
(even though Europe is the most densely populated continent). The average density of less 
developed countries is more than twice as great as the average in developed countries. And both 
east and south Asia seem likely to overtake Europe in density within a decade or so. Of course, 
we must be cautious in picturing densities in terms of people per square kilometer because of the 
human tendency to gather in clusters. Although the United States in 1975 had about 23 people 
per square kilometer on the average (about 27 per square kilometer in the coterminous forty- 
eight states), there were many square kilometers that were uninhabited. Furthermore, people 
ordinarily are not uniformly distributed within any given square kilometer. Some samples of 
population densities in 1972, both moderate and extreme, were: — 

People per 

People per 



















Earth (land area) 

United States 




New York City 

Manhattan Island 

The densities and distributions of populations, especially in relation to resources, have played 
critical roles in many important events in human history. Densities that are perceived as high by 
the members of populations themselves generate what is commonly called population pressure. 
Overpopulation is usually perceived in relation not to the absolute size of a population but to its 
density and to the resource base on which it depends. On many thousands of occasions in 
prehistory, one tribe or another must have decided that it had nearly exhausted the 

United Nations, Concise report. 

United Nations, Statistical yearbook, New York, 1973; U.S. Department of Commerce, 
Statistical abstract of the United States, Washington, D.C., 1974. 

pwumriON density 

Pf P> SQUWE KlDMfTtft. 1f« 

I -JO 


FIGURE 5-27 
Patterns of population density, 1972. Numbers represent persons per square kilometer. A square 

kilometer is 0.3861 square miles. (Data from UN Statistical yearbook, 1973.) 
TABLE 5-13 
Land Area and Population Density in the World and Major Areas, 1960 and 1975 


World total 

More developed regions 

Less developed regions 



Land area 



(1000 km 2 ) 

















U.S. and Canada 


South Asia 

East Asia 


Latin America 

"Not including the Antarctic continent. 

Source: United Nations, Concise report. 






(1000 km 2 ) 
























berries and game in its home territory and therefore moved in on its neighbors. Many famous 
migrations in history, such as the barbarian invasions of Europe in the early Christian era, may 
have been partly due to population pressures. In 1095 when Pope Urban II preached the First 
Crusade, he referred to the advantages of gaining new lands. The Crusaders were mainly second 
sons who had been dispossessed because of a growing trend toward primogeniture in Europe 
(inheritance by the firstborn son only). Considerable evidence, such as indications of attempts at 
land reclamation, suggests that population pressures were building up in Europe well before the 
fifteenth century. 

Before the arrival of Europeans in what is today the continental United States, the population 
density was 



about 0.13 people per square kilometer. The addition to Europe of the sparsely populated New 
World at the end of the fifteenth century in effect reduced the overall population density of 
Europe from about 10 people per square kilometer to fewer than two people per square kilometer 
in the total area, and at the same time enormously expanded Europe's resource base. 

European exploitation of the spatial, mineral, and other material wealth of the New World led to 
the creation of a basic set of institutions attuned to frontier attitudes. The ensuing economic 
boom lasted for 400 years. ^_At least as far as land is concerned, the boom is now plainly over. 
The population density of the European metropolis (western Europe and the Western 
Hemisphere) increased until it again exceeded 10 people per square kilometer (26 people per 
square mile) before 1930. Since all the materials on which the boom depended came ultimately 
from the land, the entire boom is clearly limited. Many of humanity's present difficulties are 
related to the rate at which the limits are being approached. And many of the lingering 
institutions and attitudes that evolved in a frontier setting now constitute major threats to the 
survival of the human population. 

Population pressure has also been observed to lead to conflict. Many wars were fought by 
European nations as they scrambled to occupy the Western Hemisphere. They warred among 
themselves and against the technologically less advanced native populations in the New 

W. P. Webb, The great frontier . 
-229- PAGENUMBER230 

World. More recently, in the 1930s and 1940s, population pressure contributed to Nazi 
Germany's famous drive to lebensraum (literally, room for living), especially in the East, where 
it reached its climax in Operation Barbarossa — the ill-fated invasion of the Soviet Union. 
Historian D. L. Bilderback has commented that in the early years of Hitler's power, "large 
numbers of intelligent and humane persons believed that the Eastern adventure was a matter of 
necessity for their own survival." Whether Germany in 1941 was overpopulated in some absolute 
sense is not the point. The nation perceived itself as overpopulated. 

Germany is probably more pressed for space today than it was then. The Bonn government, 
however, in contrast to Hitler, is not calling attention to overpopulation as a problem. Indeed, 
only recently has it begun to realize that by many standards Germany today is overpopulated. 
During the 1950s and 1960s, West Germany was importing workers in large numbers from 
southern Europe and North Africa. Today that immigration has been reduced to a trickle, and 
fertility has declined to below replacement level in both East Germany and West Germany. 

Japan's expansionist moves in the 1930s and early 1940s can be traced in part to the high 
population density on its small islands. The population growth of Japan in the last third of the 
nineteenth century and the first third of the twentieth century was unprecedented among 
industrialized nations. It doubled in size (from 35 million to 70 million), and therefore in density, 
during the sixty-three years between 1874 and 1937. When the attempt to conquer additional 
territory failed and population growth continued to accelerate, Japan legalized abortion and 
drastically reduced its population growth. But, with 111 million people in 1975 and still growing 
by 1 .3 percent per year, ^_Japan is again feeling the pinch and is looking more and more toward 
the continent of Asia for at least economic lebensraum. 

Population pressures are certainly contributing to international tensions today. The USSR, India, 
and other neighbors of overpopulated China have been guarding their frontiers nervously since 
before 1960. Population growth in China has left her few long-range choices but to expand, to 
starve, or to implement stiff population-control policies. Since the 1960s, China has followed the 
last course, but whether it will succeed remains an open question. 

Australians are clearly apprehensive about the Asian multitudes. This attitude for a long time 
was reflected in their nation's "whites only" immigration laws and Western-oriented foreign 
policy. The racial bias of Australian immigration policies was eliminated in the 1960s, but soon 
afterward there was a movement to stop encouraging immigration altogether. Australians have 
reason to be fearful of too much immigration. Because there is a generally unfavorable and 
unreliable climate over much of Australia and because of the nation's history of disastrous 
agricultural practices, the continent, although large, lacks the resources for absorbing even a 

single year's increment to the Asian population. Such an addition would more than quintuple 

Australia's population, from 13.8 million to 74 million. The number of people added annually to 
India's population alone (13 to 15 million) is more than the entire population of Australia today. 

Population density has increased in all areas of the world and can be expected to continue 
increasing as the human population grows. But the varying rates of growth from region to region 
mean that some regions may not even double their population density in the next century 
(assuming growth follows projected paths), while others will increase their density from three- to 
fivefold. Table 5-14 shows projected density from 1925 to 2075 for various regions according to 
the United Nations medium-variant projection. South Asia, for instance, is projected to reach a 
density nearly three times as high as Europe's today, which is the highest ever known previously 
in a major region. And Europe today is by no means self-sufficient for resources, or even for 


Migration is almost as characteristic of Homo sapiens as toolmaking. Perhaps only a few 
domestic animals are as widely distributed on Earth's land areas as people are. 

Population Reference Bureau, Inc., 1975 Population data sheet. 
68 A. J. Marshall, ed., The great extermination, Heinemann, London, 1966. This book 
documents past abuse of the fragile Australian environment. 
-230- PAGENUMBER231 

TABLE 5-14 

Land Area and Inhabitants per Square Kilometer, in the World and Major Areas, 1925- 

2075 (United Nations medium variant) 

Land area 

In h abitan ts/km ' 


Major area 

(1000 km 2 ) 

7925 1950 1975 2000 2025 205C 

' 207 



















U.S. and Canada 



























East Asia 


















Latin America 


















South Asia 


















Source: United Nations, 

Concise report. 

From their probable origins in Africa, human groups had spread out to occupy all the major land 
areas of the planet but Antarctica by 20,000 years ago, long before the beginnings of agriculture 
or written history. There is little question that the human habit of wandering has been a major 
factor in cultural evolution, as new ideas, and especially technical innovations, have been carried 

from one area to another. Agriculture, as one very important example, was transmitted largely 
through migrations and often by displacement of resident nonagricultural peoples. Today the 
only remaining groups of people who do not practice agriculture are a few isolated tribes in 
remote areas where a harsh climate is generally unsuitable for raising crops, such as arctic 
Eskimos, Bushmen of the Kalahari, aboriginal tribes in Australia's interior deserts, and the 
Indians of Tierra del Fuego. And all these groups are under severe pressure from modern 
civilization to abandon their hunter-gatherer cultures. 

The impetus for the movement of people in large groups may have several sources, including 
overpopulation and resource pressure, as discussed earlier. Demographer Kingsley Davis argues 
that differences between groups in levels of technology and economic opportunity are also major 
causes of migration. ^Differential technology can work both ways: people with more 
sophisticated technology may invade and conquer new areas, or less advanced groups may be 
attracted to the greater opportunities provided by a more developed society. Ancient Romans, for 
example, conquered vast areas in Europe, North Africa, and western Asia, seeking mineral 
wealth and an expanded food base. But, unlike the earlier Greeks and Phoenicians, the Romans 
did not colonize those regions in any great numbers. Rather, people from the outlying areas 
migrated to Rome, attracted by the greater economic opportunities there. Perhaps even greater 
numbers went to Rome involuntarily as slaves. 

Waves of Asian invaders swept across Europe during the Roman Empire, as they had for 
centuries before. Perhaps they were partly impelled by population pressure in Asia, but probably 
they were also attracted by the wealth of the Mediterranean basin. Migrant barbarians were 
gradually assimilated into the Roman population, and, as that civilization gradually declined, the 
technological differences between the barbarians outside and the Romans inside probably faded. 
Certainly the fall of Rome in the fifth century suggests that, in warfare at least, the barbarians 
were equal to the Romans. 

In the Middle Ages, the known major migrations were those of Islamic peoples — Arabs and 
Turks, some of whom in turn were responding to pressure from Genghis Khan's Mongols. But 
the discovery of the New World by Europeans led to a burst of exploration and exploitation 
followed by waves of migration from the seventeenth through the twentieth centuries. And, even 
as Europeans began to settle the sparsely inhabited new lands, they 

69 The migrations. 

were also claiming, colonizing, and exploiting morepopulated territories in Asia and Africa. 
Slavery and indentured labor accounted for much of the migration between 1450 and 1870, 


including movement of as many as 20 million people. — 

Migration of Europeans to the New World, Australia, New Zealand, and South Africa remained 
at fairly low levels until the nineteenth century. Davis attributes the great acceleration of 
movement in that century to the Industrial Revolution (which widened the technological gap 
between Europe and the other continents and stimulated the search for raw materials) and to 

growing economic uncertainty in Europe. The early nineteenth century in northwestern Europe 
also was a period of declining death rates and rapid population growth. England, Ireland, and 
(later) Germany and Scandinavia sent millions of migrants to North America, Australia, New 
Zealand, temperate South America, and South Africa. Between 1840 and 1930, at least 52 
million people left their home continent, most of them for North America. Large as this 
emigration was, it had little effect in retarding population growth in most of Europe, although it 
fueled population explosions in the receiving countries. In most European countries, emigration 
appears to have allowed a delay in the beginning of the birth-control movement. The Irish potato 
famine provided a special stimulus to emigration in the 1840s and 1850s, as 2 million Irish left 
the country in four years. Emigration continued at a high level, and Ireland is the only European 
country whose population growth was reversed by it. Had the same fertility prevailed without 
that safety valve, the Irish population would be more than 12 million today, rather than 3 million. 

It is fashionable to suppose that the era of large-scale migration ended before World War II, but 
that is not the case. Despite the establishment of immigration restrictions and quota systems in 
the 1920s and 1930s in many receiving countries (including the United States), migration has 
continued briskly, although there are some important differences in who migrates where. One 
significant change is the twentieth-century phenomenon of political refugees, who have attended 
each war, major or minor. One can list numerous examples: Turkish Armenians and White 
Russians early in the century; European Jews after World War II (not to mention millions of 
"displaced persons" of various national origins who settled elsewhere after that war), Palestinian 
Arabs, Chinese, Hungarian "freedom fighters," Cubans, and, in 1975, 150,000 Vietnamese. 

Another form of involuntary migration is the expulsion or exchange of minorities by nations. 
Examples include the Sudeten Germans repatriated from Czechoslovakia after World War II, and 
the Moslems and Hindus exchanged when India and Pakistan were partitioned. Davis has 
estimated that altogether more than 70 million people were displaced between 1913 and 1968. 

Migration by choice is also proceeding, but there have been changes. Northern and central 
European countries, which formerly sent emigrants to new lands, since World War II or even 
earlier have been receiving more migrants than they have sent out. At first the immigrants, 
imported to meet labor shortages, came mainly from southern European countries — Portugal, 
Spain, Italy, Greece, and Yugoslavia. More recently immigration from central and North Africa, 
India, and Pakistan has increased. Here again, as Davis points out, because the technological gap 
has widened betweed DCs and LDCs since World War II, migrants from less highly developed 
societies are moving to more developed ones in search of better jobs. Other possible factors are a 
reversal of relative population growth rates (earlier in the century, Western countries were 
growing more rapidly than non-Western countries) and the recent independence of former 
colonies. A part of this picture, too, is the brain drain, in which professionally trained citizens of 
LDCs, unable to find satisfactory employment at home, move to DCs. 

A similar change in migration patterns has taken place in the United States. Before 1890, 
immigrants to the United States came predominantly from northern and western Europe. After 
the turn of the century, most came from southern Europe. The 1965 Immigration Act abolished 
the national quota system, and since then immigrants from LDCs, particularly Latin America, 
have far outnumbered Europeans. 71 Figure 5-28 shows 

70 Ibid. 

7 1 

Charles B. Keely, Immigration composition and population policy. 
-232- PAGENUMBER233 

the national origins of immigrants to the United States in 1972. While national quotas no longer 
apply, there are quotas for people based on other criteria: relatives of citizens or residents, skilled 
or unskilled workers in desired occupations, and refugees (except in unusual situations like the 
Cuban revolution and the Vietnamese evacuation). Hence, age and sex compositions of 
immigrants as a group have changed, as have their countries of origin. There has been an 
increase in the proportions of professionals, of married people with families, and of males rather 
than females since 1965. 

The total number of immigrants admitted annually to the United States has been about 400,000 
in recent years. Unfortunately, no useful information exists on how many have left the country 
permanently, so the actual contribution of net migration to the population growth of the United 
States is unknown. Usually, emigration is ignored in discussions of United States population 
dynamics or immigration policies. 

During the nineteenth and the early twentieth centuries immigration was an important element of 
United States population growth. Between 1900 and 1910, it accounted for more than half the 
nation's growth. It reached a low of less than 6 percent in the 1930s and 1940s, but by 1973 it 
again comprised more than a quarter of United States population growth (see Table 5-15 ). 

The above discussion is based only on legal immigration. But the United States also receives 
large numbers of illegal immigrants, mainly from Mexico (estimates range from 60 to 85 percent 
of illegal immigrants) but also from several other Latin American and Caribbean countries, the 
Philippines, China, Korea, Nigeria, Ethiopia, Italy, Greece, Iran, and some other Middle Eastern 
states. - 

Although there is no way to know exactly how many illegal immigrants are now residing in the 
United States or how many enter each year, some estimates have been made. It is believed that 
by 1974 some 8 million illegal aliens were living in the United States, _ despite the 













Trinidad and 





50 60 



FIGURE 5-28 

Sources of recent immigration to the United States are ranked here in descending order, with the 

top 1 countries of origin in the Western Hemisphere distinguished by shade from the top 1 in 

the Eastern Hemisphere. The figures for each country in the Western Hemisphere (except Cuba) 

represent the number of visas issued in fiscal year 1972; the figure for Cuba includes 16,380 

"adjustments of status" granted by the Immigration and Naturalization Service to aliens subject 

to numerical limitations. The figures for the Eastern Hemisphere represent the number of visas 

issued in fiscal year 1972 (excluding recaptured visas) together with the number of adjustments 

of status and conditional entries granted to aliens subject to numerical limitation. (Data from 

U.S. State Department, 1973; from Davis, 1974.) 


A. C. McLellan and Boggs M. D., Illegal aliens: A story of human misery; "The alien wave", 
Newsweek, February 9, 1976. 

"The alien wave", Newsweek, February 9, 1976. This was confirmed by a study made by the 
Census Bureau in 1975, reported by CBS radio, November 7, 1975. 
-233- PAGENUMBER234 

TABLE 5-15 

Ratio of Immigration to Population Growth in the United States, 1820-1973 



Immigration as 

Decade ending 




% of growth 









































































Source: John Tanton, ZPG and Immigration: A discussion paper ( Zero Population Growth, Inc., 

Washington, D.C., 1974 (from data of the U.S. Bureau 

of the Census and U.S. Immigration and Naturalization Service). 

deportation of hundreds of thousands each year (more than 600,000 in 1973 and 800,000 in 
1974). Estimates of annual clandestine arrivals range from 400,000 to 3 million. Illegal 
immigrants have often been encouraged to enter the country by prospective employers, covertly 
or overtly, to take very low-paying, menial jobs that are nevertheless more attractive than what is 
available in their home countries. Many jobs now held by illegal aliens are in urban areas, and 
some require highly skilled workers. The influx of foreign workers causes problems as they 
compete for jobs with welfare recipients and other low-income citizens, thus depressing the 
United States wage scale and sending funds out of the country. All of these problems were 
exacerbated by the recession of 1974/ 1975. 

The demographic impact of illegal immigrants is bound to be large if they continue to arrive at 
the current rate. Even at the lowest estimated level of entry, their numbers equal those of legal 
immigrants. Demographic projections made by the organization Zero Population Growth have 
indicated that, if immigration proceeded at the rate of 1 .2 million per year (400,000 legal and 
800,000 illegal immigrants), and fertility remained at the 1975 level of 1.9 children per woman, 
the United States population would soar to 346 million in 2050. — 

Like the unknown numbers of emigrants who leave the United States, illegal immigrants are left 
out of most estimates of United States population size and growth (with the major exception of 
the World Population Estimates of Robert C. Cook for the Environmental Fund). Efforts to pass 
legislation in Congress to strengthen regulations against illegal immigrants and to penalize 


employers for hiring them have failed so far. Illegal immigrants do not enter the United States 

only; several countries in Europe and South America also attract them, mainly for the same 

Migration, of course, can have no effect on the growth of population worldwide; there is no 
migration to or from Earth. But it can have significant effects on the rates of growth of individual 
countries, on their age compositions, and on population densities. 75a Migration obviously 
increases the growth of receiving countries and reduces that of sending countries. In the case of 
the United Kingdom, mentioned above, net emigration produced negative population growth by 
1975, even though the age composition and fertility rates should have produced a slow positive 
growth and immigrants from less developed former colonies were still being admitted. 76 But the 
number of immigrants had been reduced by tightened regulations, and the number of emigrants 
(mostly native-born) sharply rose in response to deteriorating economic conditions. Most 
emigrants went to Australia, Canada, and New Zealand. 

Emigration from rapidly growing LDCs also has the effect of dampening population growth in 
the country of origin, perhaps providing enough of a safety valve to delay establishment of 
needed population programs. 

74 Melanie Wirken, "Illegals threaten U.S. population stabilization", Intercom (Population 
Reference Bureau, Inc.), vol. 6, no. 4, June 1976, p. 4. 


"The enterprising border jumpers", Time, May 19, 1975; The alien wave. 
75a For a discussion of these effects and related consequences, see John Tanton , International 



InternationalJournal of Environmental Studies, p. 300, n.52. 

This appears to have been the case for Mexico, which only established a family-planning 
program in 1974, although the nation's birth rate has been around 46 per 1000 for some time. 
Similarly, several Caribbean islands ( Puerto Rico, Barbados, and Trinidad, for example) have 
for decades exported substantial fractions of their natural increase, mainly to the United States or 
Canada. Birth rates have fallen in several Caribbean nations since 1965, but the possibility 
remains that they might have fallen considerably sooner and faster without the safety valve of 


One of the oldest of all demographic trends is the one towards urbanization. Preagricultural 
people by necessity were dispersed in small groups over the landscape. Hunting and gathering 
required perhaps a minimum of 5 square kilometers of territory to produce the food for one 
person. Under such conditions and without even the most primitive transportation systems, it was 
impossible for people to exist in large concentrations. But the Agricultural Revolution changed 
all that. Because more food could be produced in less area, agricultural people began to form 
primitive communities. The ability of farmers to feed more than their own families was 
obviously a prerequisite of urbanization. A fraction of the population first had to be freed from 
cultivation of the land before cities could develop. 

But the division of labor and specialization in a nonfarming population does not seem in itself to 
have led immediately to urbanization. For example, some scholars believe that Egyptian 
agriculture and society were such that considerable numbers of people were freed from the land 
by the time their culture reached the stage usually considered civilized (about 3000 B.C.). But it 
was another 2000 yews before they developed the kind of complex, interrelated society 
recognizable as a city. Similarly, it is doubted whether the Mayan civilization produced cities. It 
is thought that, in ancient Mesopotamia, at least, and perhaps in Cambodia, the development of 
large and complex irrigation systems helped lead to the formation of cities. Then, as today, 
shortages of water formed the bases of political disputes, so people may have gathered in large 
groups for defensive purposes. Mesopotamian cities also would have served as storage and 
redistribution centers for food. As anthropologist Robert M. Adams has written, "the complexity 
of subsistence pursuits on the flood plains may have indrectly aided the movement toward cities. 
Institutions were needed to mediate between herdsman and cultivator; between fisherman and 


sailor; between plowmaker and plowman." It has also been suggested that mobilizing, 

working, and using metals may have stimulated concentrated settlement patterns. Whatever the 
actual impetus for urbanization, the first cities arose along the Tigris and Euphrates rivers 
between 4000 and 3000 B.C. 

The trend toward urbanization continues today, as it has ever since those first cities were formed. 
Because of differing fertility patterns and generally much poorer health conditions, cities 
traditionally have been unable to maintain their populations through natural increase, let alone to 
grow. Hence, growth of cities before the late nineteenth century was almost entirely due to 
migration from rural areas. The move to the cities has at times been stimulated by agricultural 
advances that made possible the establishment of larger, more efficient farms. It seems also to 
have been stimulated by population growth in rural areas, which necessitated either the 
subdivision of farms among several sons or the migration of "surplus" offspring to the cities. In 
the past, advances in agriculture generally have been accompanied by advances in other kinds of 
technology, which provided new opportunities for nonagricultural employment. Beyond 
providing places for those displaced from the land, cities have always been attractive in 


themselves to people who hoped to improve their economic conditions. — 

The movement into large urban concentrations has been especially rapid in the past century. For 
instance, in the United States about 6 percent of the population lived in urban areas in 1800,15 
percent in 1850, and 40 percent in 1900. Today, nearly 75 percent of the population lives in cities 

or their suburbs ( Figure 5-29 ) . Recent Census Bureau evidence indicates, however, that the 
trend to 

R. M. Adams, The origin of cities. 

For a fascinating study of the history of cities, read Lewis Mumford, The city in history: Its 
origins, its transformations, and its prospects. 

1T9Q160Q 10 20 30 &0 1850 6C 

I90O '0 20 30 *0 T950 60 70 

N - 


I 1 NO m*>upo<tj|fl populoKFi 


urbanization in the United States may have ended — and may even have begun a reversal. 


Other industrialized nations are also more or less heavily urbanized, ranging from 56 percent in 
eastern Europe to 86 percent in Australia and New Zealand. Most LDCs are much less 
urbanized, but the variation among them is even greater. Tropical South America reaches 
European levels ~ 59 percent ~ whereas East Africa is only 12 percent urbanized. The trend to 
urbanization is accelerating rapidly in those countries, however, attended by a variety of severe 
social problems. Altogether, almost 40 percent of the world's population was urban in 1970. — 
Table 5-16 traces the urbanization of major regions since 1925 and projected to 2025. 

An additional change is in the proportion of the world's people who live in very large cities 
(those with 1 million or more inhabitants). A generation ago most such cities were in developed 
countries. By the 1970s more of the million-cities were in LDCS, and their aggregate population 
exceeded that in DCs. Table 5-17 shows the growth of million-cities between 1960 and 1975. 

One problem that is inevitably encountered when discussing urbanization is the definition of an 
urban area. This varies from country to country, and from time to time. Furthermore, urban areas 
in different countries or different areas of the same country often are quite dissimilar. Los 
Angeles, New York, and Chicago have certain features (good and bad) in common-good art 
museums, major universities, slums, disadvantaged minorities, numerous TV stations, clogged 














































streets and freeways, dangerously congested airports, diverse specialty shops, high crime rates, 
and air pollution, to name a few. But their differences are as apparent as their similarities. The 
air-pollution problems of Los Angeles 

TABLE 5-16 Percentage of Total Population Living in Urban Areas in the World and 
Major Areas, 1925-2025 

Area 1925 1950 1975 2000 2025 


U.S. and Canada 



East Asia 

Latin America 


South Asia 


Source: United Nations, Concise Report. 

and New York differ fundamentally because the smogs over the two cities have different 
compositions and the cities are in different physical settings. Water-supply problems are unique 
in each of the three cities. Los Angeles has a seemingly hopeless surface transportation problem 
and smoggy "sunshine slums." MexicanAmericans are one of its largest minority groups. 
Chicago has substantial problems with migrants from the Ozarks. New York has had great 
difficulty in satisfactorily absorbing masses of immigrants from Puerto Rico and the rural South, 
many of whom have ended up as welfare recipients. The problems of government in the three 
cities have their own peculiar twists, but all have more or less serious financial problems. 

Urbanization seems to have one almost universal effect, the breaking-down of the traditional 
cultures of those who migrate to the cities—a loss of roots, or alienation. In rural or tribal 
societies each individual has a well-defined role in the organization of the society, a role that he 
or she has matured into and that is recognized by all other members of the society. In contrast, 
anonymity is a major feature of the city. City-dwellers typically are on close terms with no more 
people than are village-dwellers, and they tend to go to great lengths to 

FIGURE 5-29 {left) Urbanization of the United States. In 1970, some 73.5 percent of Americans 

were living in towns or cities, and 68.6 percent were living in large cities and their surroundings 

(metropolitan areas). Since 1970 the percentage of urban Americans has passed 75. (A. Adapted 

from Population bulletin, vol. 19, no. 2. Courtesy of the Population Reference Bureau, Inc., 

Washington, D.C. B. U.S. Bureau of the Census, 1970.) 


Roy Reed, "Rural areas' population gains now outpacing urban regions", The New York 
Times, May 18, 1975. For a more detailed description of this new phenomenon, see Peter A. 
Morrison, with Judith A. Wheeler , Rural renaissance in America? The revival of population 
growth in remote areas. 

80 United Nations, Concise Report. 
-237- PAGENUMBER238 

TABLE 5-17 Million-Cities, 1960 and 1975 

Number of 

Population of 



Percentage of 
total population 
in million-cities 















More developed regions 







Less developed regions 





















U.S. and Canada 














South Asia 







East Asia 














Latin America 







Note: The 1975 estimates correspond to earlier population estimates and projections of the 
United Nations. 

Source: United Nations, Concise report. 

avoid "getting involved" with the vast majority of the human beings with whom they come into 
daily contact. 

Urbanization in the United States in some ways differs dramatically from urbanization in other 
countries. For instance, the difference between city-dwellers and country-dwellers in the United 
States has become increasingly blurred, especially in recent years, with urban culture becoming 
dominant. Rapid transportation and mass media have exposed the country folk to the ways of the 
city. Furthermore, in the United States especially, the phenomenon of suburbanization has 
developed. Suburbanites, who take advantage of high-speed automobiles, plentiful and cheap 
fuel, freeways, and affluence, have attempted to enjoy the advantages of city and countryside 
simultaneously, by working in the former and living close to the latter. 

Suburbanization has been extremely rapid since World War II and has led to some severe 
problems in the United States. While affluent and middle-class taxpayers have left the cities, 
poor and unskilled people squeezed out of rural areas largely by increasingly mechanized 
agriculture, have flooded to the cities seeking jobs in industry. As tax returns to city governments 
have dwindled, it has become more and more difficult for cities to maintain even basic services. 
In an effort to restore their tax losses, many cities have concentrated their urban renewal funds 
on building high-rise office buildings used mostly by commuting suburbanites, while the slum 
areas housing the poor have become even more crowded, neglected, and crime-ridden. Thus, the 

contemporary United States city consists largely of office buildings and slums, surrounded by 
affluent suburbs, freeways, and industrial areas. The central city is strangled during the day by 
traffic as suburbanites commute to the city and factory workers commute out of it. Nevertheless, 
American cities have access to the resources to solve their urban problems, difficult though they 
are. Their problems are largely the result of poor planning or no planning at all. 

European cities, too, have been suburbanized, but not to the same degree as those of North 
America. Because it has greater population density, and consequently less land to spare, Europe 
has avoided the greatest excesses of urban sprawl. Different urban government arrangements and 
zoning regulations have also played a part in keeping cities relatively compact. Moreover, 
urbanization has proceeded more slowly in Europe than elsewhere. This is not to say there are no 
urban problems in Europe. Even though public transport systems are generally far superior to 
those in the United States, traffic congestion (largely from automobiles) is almost unmanageable 
in the centers of many of the largest cities. Housing shortages have plagued European cities at 
times; public or publicly subsidized housing has made up a substantial portion of housing built 
since World War II in some countries. In London, there is a serious shortage of housing as well 
as a growing number of abandoned buildings, some slated for demolition. Homeless families by 
the thousands have asserted squatter's rights by occupying the derelict 


structures — in some cases, even with government assistance. 

While rapid urbanization has caused problems in industrialized countries, it is virtually a disaster 

o 1 

in many LDCs. There, urban problems are largely the result of cities' being overwhelmed by 

massive, unanticipated immigration from the countryside. Between 1950 and 1960, the 
populations of cities in the DCs increased 25 percent, while those of cities in the LDCs increased 
55 percent. In most LDCS, especially since the end of World War II, there has been an 
increasing flood of impoverished peasants into urban areas. Yet employment opportunities there 
have not materialized. The result has been the development of huge shantytowns around the 
fringes of cities. 

In Latin America the shantytowns are given different names in each country: favelas in Brazil, 
tugurios in Colombia, ranchos in Venezuela, and barriadas in Peru. In Peru, at least a million 
squatters live in such settlements—a substantial fraction of Peru's population of about 15 million ( 
Figure 5-30 ). They are virtually a universal phenomenon around large cities in Central America 
and tropical South America. Most of these urban migrants become permanent residents, and 
among them, women outnumber men. The majority of arrivals are between 15 and 30 years old. 

The trend in Africa has been similar, with hundreds of thousands migrating to the cities annually 
in search of better lives. Nairobi, the capital of Kenya, had a 1968 population of 460,000 and was 
growing at a rate of 7 percent per year. (That is more than twice the growth rate of Los Angeles 
in the decade 1950 to 1960.) During the 1960s, Accra, the capital of Ghana, was growing at 
almost 8 percent per year; Abidjan, capital of the Ivory Coast, at almost 10 percent; Lusaka, 


capital of Zambia, and Lagos, capital of Nigeria, both at 14 percent. Unlike Latin American 

urban migrants, African migrants are usually young men who stay in the city only temporarily 

and then return to their families in the countryside. Lack of employment and housing for families 
are probably the reasons for the turnover. 

FIGURE 5-30 A shantytown in the Rimac district of Lima, Peru. Many squatter houses, 

originally straw shacks, are being rebuilt in brick and masonry whenever the earnings of the 

owners permit. (Photo courtesy of William Mangin.) 

81 Atlas report, Cities in trouble, Atlas, December 1975. See also George J. Beier, Can third 
world cities cope? for a concise analysis of the situation. 
"Bulging African cities where dreams die", San Francisco Chronicle, November 20, 1968. 


The rate of urbanization in Asia has also been rapid in this century, but in most Asian countries 
the increases have been from a rather low base. For instance, at the turn of the century about 1 1 
percent of India's population was urban. Today more than 20 percent of India's people live in 
cities. This deceptively low rate of urbanization conceals what for India is a problem of immense 
proportions ( Figure 5-31 ) . As in Africa, many of India's (and other Asian) urban migrants 
remain in the cities only temporarily. 

Some of the world's fastest- growing cities are in southeast Asia, especially in Indonesia. Hong 
Kong and, to a lesser degree, Singapore have had rapid urbanization compounded by high rates 
of immigration — often illegal — from China. Although both cities have prospered as 
manufacturing and trading centers, they have succeeded in providing housing and services for 
their burgeoning populations only through strenuous efforts. Table 5-18 shows the projected 
growth of the world's largest and fastest-growing cities between 1970 and 1985-only fifteen 
years. In that time, many will double their populations and one ( Bandung, Indonesia) will more 
than triple its population. 

In most rapidly growing LDCS, the trend to the cities seems to be caused in large part by the 
same hope for a better life that has drawn people from rural areas of the southern United States 
and Puerto Rico to a slum life in New York, Chicago, and other northern metropolises, or from 
rural southern Europe to industrial northern Europe. But in the cities of less developed countries, 
where there is little industry, economic opportunities are much more limited than in the cities of 
the United States or Europe. 

In the LDCS, communications and transportation are much less efficient than in DCs, and the 
peasant cultures are less influenced by the urban. The overwhelming majority of urban-dwellers 
in LDCs are migrants from the countryside who have brought their peasant cultures with them. 
Unlike the majority of DC urbanites, whose specialized education, training, and skills assure 
them of places in the city's complex social web, the LDC immigrant has no such skills to offer. 
In the United States, unskilled rural immigrants are in the minority. 

Although they may have employment problems, most are at least literate and can be absorbed 
into the industrial society. 

Cities in developed countries are a source of wealth and power, generated through technology 
and manufacturing. The goods they produce are exchanged for raw materials and food from the 
countryside. In contrast, many LDC cities subsist in times of shortage primarily on food 
imported from other countries. Attracted by the opportunity to obtain a share of the imported 
food, inhabitants of the countryside move into the cities when the countryside can no longer 
support them. Inevitably, they find that their limited skills render them incapable of contributing 
to the economy. As a consequence, they are not much better off than they were where they came 
from. In some LDC cities, squatters now make up a majority of the population. According to the 
United Nations, they constitute at least one-third of the urban populations of all less developed 
areas and are increasing by about 15 percent per year — doubling every five or six years. — 
Miserable as their condition seems to be, at least those who have settled with their families 


evidently prefer to remain in their squatter settlements rather than return to what they left. Many, 
of course, may have burned their bridges behind them and have no way of successfully returning 
to their former homes. Many migrants to LDC cities do, however, maintain contact with their 
home villages and contribute financially to their rural relatives. 

Evidence is accumulating that many squatter settlements are far more successful as a way of life 
than would appear to Western eyes. Among the permanent migrants, modified village societies 
are often established within squatter settlements, thus transferring village culture to the city (a 
factor that may explain why their attitudes and reproductive patterns generally resemble those of 
rural people). Such established settlements are very different from rural slums in the United 
States or other DCs, which are often characterized by an absence of community coherence and 
efforts toward neighborhood improvements. 

Squatter settlements often start out with the most 

N. Keyfitz, Population density and the style of social life. 
84 United Nations, Report on the world social situation, Center for Housing, Building and 
Planning, Department of Economic and Social Affairs, New York, 1974. 

TABLE 5-18 Growth of the World's Cities, 1970-1985 



1985 population 

Projected growth 





1 New York 




2 Tokyo 




3 London 




4 Shanghai 




5 Paris 




6 Los Angeles 




7 Buenos Aires 




8 Mexico City 




9 Sao Paolo 




10 Osaka 




1 1 Moscow 




12 Peking 




13 Calcutta 




14 Rio de Janeiro 





1 Bandung 




2 Lagos 




1970 population 

1985 population 

Projected growth 




3 Karachi 




4 Bogota 




5 Baghdad 




6 Bangkok 




7 Teheran 




8 Seoul 




9 Lima 




10 Sao Paulo 




1 1 Mexico City 




12 Bombay 













Source: Adapted from People, vol. 1, no. 4, p. 10. 

rudimentary of dwellings but progress to more permanent structures as their owners can afford to 
build them. The shantytowns are built by their inhabitants, who organize a community social 
structure in order to bring in needed services from outside, such as water, food, public 

transportation, schools, and health care. This pattern is especially common in Latin America. 

In some of the more successful cities, governments have cooperated with the settlements in 
providing these services and assisting the people in their community projects. Around Lima, for 
instance, settlements are being steadily improved through the joint efforts of the squatters and the 
Peruvian government. ^_Such cooperation among governments and squatters is relatively rare, 
unfortunately. More commonly, governments try to ignore the shantytowns, or — more 
ambitiously — try to clear the slums and thereby "remove" the problem (without success). Where 
slum clearance has been attempted, the result has often been social disaster — or at least an 
extremely expensive program of building urban housing for the poor. — 

W. Mangin, Squatter walements. 

Sally E. Kellock, a UNICEF (United Nations Children's Fund) program officer in Peru, 
personal communication. 

United Nations, 1974 Report on the world social situation. 

FIGURE 5-31 A street in Calcutta. These people live in makeshift shacks built on the sidewalk. 
Some of the shacks also function as shops where the "owners" sell food and handmade articles. 

(Wide World Photos.) 

One of the major obstacles to improving conditions for urban squatters is the apparent inability 
of governments to find solutions in any but traditional Western approaches. Just one outstanding 
example is the sanitation facilities — or lack of them ~ in most LDC cities. Sewer systems serve 
only 28 percent of the LDC urban populations; 30 percent of those populations have no facilities 

at all. Calcutta, for instance, relies on a crumbling sewer system built at the turn of the century 

for 600,000 people — less than one-tenth of the present population. Calcutta first tried slum 

clearance, but has since turned to making modest improvements in the bustees (as the 
shantytowns are called), providing latrines and minimal water service. 90 Most LDCS, however, 
seem unable to conceive of sanitation except in terms of unsanitary latrines or western-style 
sewer systems, which are well beyond their means, technologically and financially, and which 

use too much water. So nothing is done. A program is urgently needed to develop intermediate 
technologies to solve this problem-perhaps along the line of the clivus multrum dry toilet (see 
Chapter II). 

Urban projections. Even more alarming than the present urban situation are projections of 
trends in urbanization. For instance, one projection has led to an estimate for Calcutta in the year 
2000 of 66 million inhabitants, more than 8 times today's population. Needless to say, that total 
will not be reached — but there is a realistic expectation that the population of this teeming city ( 
Figure 5-31 ), in which several hundred thousand people live in the streets today, will increase 
from 7.5 million to 12 million by 1990. Calcutta is already a 

Richard Feacham, Appropriate sanitation. 

Peter Wilsher, "Everyone, everywhere, is moving to the cities", The New York Times, June 22, 
90 Atlas report, Cities in trouble. 
-242- PAGENUMBER243 

disaster area, and the consequences of further growth at such a rate are heartrending to 

The population of relatively prosperous Tokyo is projected to reach 40 million in the year 2000 
(compared to 15 million in 1970). In a desperate attempt to create land for expansion, Tokyo has 
been using 7000 tons of garbage a day to fill Tokyo Bay. Flat, empty land is at a premium in 
mountainous, overpopulated Japan. Middle-class apartments are already so scarce that people 
have to wait two years to obtain them. Tokyo's dense crowding seems destined only to get worse. 

Demographer Kingsley Davis has made some extrapolations of urbanization trends and has 
produced some startling statistics. If the urban growth rate that has prevailed since 1950 
continues, half of the people in the world will be living in the cities by 1984. If the trend should 
continue to 2023 (it cannot!), everyone in the world would live in an urban area. Most striking of 
all, in 2020 most people would not just be in urban areas; half of the world's human beings 
would be in cities of more than I million population, and in 2044 everyone would exist in "cities" 
of that size. At that time the largest "city" would have a projected population of 1.4 billion 
people, out of a projected world population of 15 billion. ^_(The word city here appears in 
quotation marks because, should such a stage be reached, world living conditions would make 
the term meaningless in its historical sense.) 

Such projections, of course, are merely extensions of recent trends. They are helpful in showing 
the direction in which Homo sapiens is headed. But, if anything, they make it clear that those 
trends cannot long continue. The movement to urbanization in the developed world may indeed 
be ending now. In much of Europe it has markedly slowed in the past decade, and in the United 
States it appears to have reversed slightly in the 1970s. 

How long the cities of less developed countries can go on absorbing masses of unemployable 
peasants in makeshift slums without major social breakdown is an open question. Much probably 

could be done to alleviate their explosive urban growth by concentrating development effort in 
rural areas and smaller towns in order to provide useful employment there. But the key to solving 
the problem in the long run is clearly to reduce the rate of population growth as quickly as 
feasible. From at least one point of view, burgeoning LDC cities are both symptomatic and 
symbolic of humanity's predicament. There one finds juxtaposed many of the contradictions of 
today: wealth and poverty; modern technology and industry and illiteracy; abundance and 

The future of the human population depends on much more than population dynamics. Those 
dynamics are limited by the physical, biological, and social environments in which the 
population finds itself. Furthermore, human actions can change both kinds of environments for 
better or worse. These interactions are explored in some detail in the following chapters. 

91 The urbanization of the human population. 
-243- PAGENUMBER244 

Recommended for Further Reading 

Bogue, D. J. 1969. Principles of demography. Wiley, New York. A good basic text. 

Coale, A. J. 1974. "The history of the human population", Scientific American, September, pp. 
41-51. A concise account of demographic history. 

Davis, K. 1965. "The urbanization of the human population". Scientific American, September, 
pp. 41-53. (Scientific American offprint 659, W. H. Freeman and Company, San Francisco.) 
Useful article on development of early cities and modern trend toward urbanization. 

1974. "The migrations of human populations". Scientific American, September, pp. 93-105. 

Informative and interesting discussion of causes and consequences of migration. 

Dumond, D. E. 1975. "The limitation of human population: A natural history". Science, vol. 187, 
pp. 714-721 (February 28). Interesting article on ways in which various societies have controlled 
their population growth. 

Freedman, R., and B. Berelson. 1974. "The human population". Scientific American, September, 
pp. 30-39. General overview of the present population situation. 

Mumford, L. 1961. The city in history: Its origins, its transformation and its prospects . Harcourt, 
New York. Fascinating compendium on cities. 

Population Reference Bureau (PRB). Population Bulletin. PRB, Washington, D.C. This is a 
useful source of information for the educated layperson on virtually all aspects of demography. 

Annual. Population data sheet. A concise summary of population numbers and related data 

listed by country. 

Teitelbaum, M. S. 1975. "Relevance of demographic transition theory for developing countries". 
Science, vol. 188, pp. 420-425 (May 2). Suggests that demographic transition theory for Europe 
may not be applicable to LDCS. 

Thompson, W. S., and D. J. Lewis. 1965. Population problems. 5th ed. McGraw-Hill, New 
York. A basic text. 

United Nations. 1974. Concise report on the world population situation, 1970-1975, and its 
long-range implications. New York. Excellent account of present world population situation; 
much useful data. 

Westoff, C. F. 1974. "The populations of the developed countries". Scientific American, 
September, pp. 108-120. On the population situation in DCs, especially recent fertility declines 
toward replacement levels. 

Additional References 

Adams, R. M. 1960. "The origin of cities". Scientific American, September. (Scientific American 
offprint 606, W. H. Freeman and Company, San Francisco.) An interesting paper on how and 
why the first cities appeared in the Middle East. 

Adlakha, A. and D. Kirk. 1975. Vital rates in India 1961-71 estimated from 1971 census data. 
Population studies, vol. 28, no. 3. An analysis of the data from India's 1971 census. 

Atlas report. Cities in trouble. "Atlas", December 1975. Contains four articles on urban problems 
in both DCs and LDCs and some ideas for solving them. 

Behrman, S. J.; L. Corsa, Jr.; and R. Freedman, eds. 1969. Fertility and family planning: A world 
view. University of Michigan Press, Ann Arbor. Useful source for recent demographic history. 


Beier, George F., 1976. "Can third world cities cope?" Population bulletin, vol. 31, no. 4 ( 
Population Reference Bureau, Inc., Washington, D.C.). Concise Analysis of LDC urbanization 

Birdsell, J. B. 1968. Some predictions for the Pleistocene based on equilibrium system among 
recent hunter-gatherers. In Man the hunter, R. B. Lee and J. DeVore, eds., Aldine, Chicago, pp. 

Bouvier, Leon F., 1975. "U.S. population in 2000: Zero growth or not?" Population Bulletin; vol. 
30, no. 5. An analysis of census bureau projections of United States population growth. 

Brackett, J. W., and R. T. Ravenholt, 1976. World fertility, 1976: An analysis of date sources 
and trends. Population reports, series J, no. 12. (November). Recent demographic trends. 

Braidwood, R. J. 1960. "The Agricultural Revolution". Scientific American, September. 
(Reprinted in Paul R. Ehrlich et al., Man and the ecosphere.) 

Brown, Harrison; John P. Holdren; Alan Sweezy; and Barbara West, eds. 1974. "Population 
Perspective 1973". Freeman, Cooper, San Francisco. Articles reviewing recent developments in 
population around the world. See especially the articles on mainland China. 

Brown, Lester R., 1976. "World population trends: Signs of hope, signs of stress". Worldwatch 
Paper 8. Worldwatch Institute, Washington, D.C. (October). 

Carr-Saunders, A. M. 1936. World population. Oxford University Press, Fairlawn, N.J. A classic 

Cavalli-Sforza, L. L., and W. F. Bodmer. 1971. The genetics of human populations . W. H. 
Freeman and Company, San Francisco. Contains a good discussion of genetics, demography, 
population projection matrices, and such. 

Curwen, E. C, and G. Hatt. 1953. Plough and pasture Henry Schuman, New York. Early history 
of farming. 

Dalrymple, D. G. 1964. "The Soviet famine of 1932-34". Soviet Studies, vol. 14, pp. 250-284. 
An excellent and detailed account. 

Davis, K. 1956. "The amazing decline of mortality in underdeveloped areas". American 
Economics Review, vol. 46, pp. 305-318. An early paper on the success of death-control 

1965. "The population impact on children in the world's agrarian countries". Population 

Review, vol. 9, pp. 17-31. Contains details of the argument that children get a disproportionately 
bad deal in the LDCs. 

Deevey, Edward S. 1960. "The human population". Scientific American September. (Scientific 
American offprint 608, W. H. Freeman and Company, San Francisco.) Concise history of 
population growth. 

Demko, G. J.; H. M. Rose; and G. A. Schnell, eds. 1970. Population geography: A reader. 
McGraw-Hill, New York. Background information on population structure and distribution, 
urbanization, and so forth. 

Dickeman, Mildred. 1975. "Demographic consequences of infanticide in man". Annual review of 
systematics and ecology, vol. 6, Annual Reviews, Inc., Palo Alto, Calif. Fascinating article on 
motivation of family limitation-goes far beyond the phenomenon of infanticide. 

Durand, J. D. 1967. "A long-range view of world population growth". Annals of the American 
Academy of Political Science, vol. 369, pp. 1-8. 

Easterlin, R. A. 1968. Population, labor force, and long swings in economic growth. Columbia 
Press, New York. An analysis of economic influences on fertility. 

Ehrlich, Paul R., John P. Holdren, and Richard W. Holm, eds. 1971. Man and the ecosphere, W. 
H. Freeman and Company, San Francisco. 

Important papers from Scientific American with critical commentaries. 

Enke, S. 1970. Zero population growth, when, how and why. Tempo, General Electric Co., Santa 
Barbara, Calif., January. A relatively nontechnical discussion of population momentum. 

Environmental Fund, Inc. (Annual.) World population estimates. Washington, D.C. A useful data 
sheet on population. 

Feacham, Richard. 1976. "Appropriate sanitation". New Scientist, January 8. On the sanitation 
facilities in LDC cities. 

Freedman, Deborah, ed. 1976. "Fertility, aspirations, and resources: A symposium on the 
Easterlin hypothesis". Population and Development Review, vol. 2, nos. 3 and 4. Discussion of 
economic influences on fertility. 

Freedom, R. 1960. "The sociology of human fertility: A trend report and bibliography". Current 
Sociology, vol. 9, no. 1, pp. 35-119. 

Frejka, T. 1968. "Reflections on the demographic conditions needed to establish a U.S. 
stationary population growth". Population Studies, vol. 22 (November), pp. 379-397. 

1973. The future of population growth. Alternative paths to equilibrium. Wiley, New York. 

A detailed examination of population momentum. 

1973. The prospects for a stationary world population. Scientific American, March, pp. 15- 

23. Less detailed than Frejka's book, and somewhat optimistic in tone. 

1974. "World population projections: Alternative paths to zero". Population Bulletin, vol. 

29, no. 5. A good summary of the material in Frejka's book. 

Hance, W. A. 1970. Population, migration and urbanization in Africa. Columbia University 
Press, New York. 

Keely, Charles B. 1974. "Immigration composition and population policy". Science, vol. 185, pp. 
587-593 (August 16). An informative discussion of recent changes in United States immigration 
policies and their consequences. 

Keyfitz, N. 1966. "How many people have ever lived on Earth?" Demography, vol. 3, pp. 581- 
582. An estimate that 4 to 5 percent of all human beings ever born we alive today. 

1966. "Population density and the style of social life". BioScience, vol. 16, no. 12 

(December pp. 868-873. Information on the origin of cities and differences between DC and 
LDC cities. 

1971. "On the momentum of population growth". Demography, vol. 8, no. 1 (February pp. 

71-80. Source of the "Keyfitz numbers." 

and W. Flieger. 1971. Population: Facts and methods of demography. W. H. Freeman and 

Company, San Francisco. Gives life tables and other calculations for most countries where birth 
and death statistics exist, and explains methods used in those calculations. Age distributions, sex 
ratios, and population increase we among its themes. Highly recommended. 

Kolata, G. B. 1974. "!Kung hunter- gatherers: Feminism, diet and birth control", Science, vol. 
185 September, pp. 932-934. Provocative discussion of birth control among hunter-gatherers. 

Langer, W. L. 1958. "The next assignment". American historical review, vol. 63, pp. 283-305. 
Includes an excellent discussion of the long-term effects of the black death on European society. 

1964. "The black death". Scientific American; February. (Scientific American offprint 619, 

W. H. Freeman and Company, San Francisco. Reprinted in Paul R. Ehrlich et al., Man and the 
ecosphere.) Interesting historical study of the plagues in medieval Europe. 


1972. "Checks on population growth: 1750-1850". Scientific American, February, pp. 92- 

99. Fascinating account of the prevalence of infanticide in nineteenth-century Europe. 

1975. "American foods and Europe's population growth, 17501850". Journal of Social 

History, winter, pp. 51-66. 

Lee, R. B. 1972. Population growth and the beginnings of sedentary life among the !Kung 
bushmen. In population growth: Anthropological implications, B. Spooner, ed. The M.I.T. Press, 
Cambridge, Mass. On how hunter-gatherers change their reproductive behavior when they take 
up agriculture. 

McKeown, T.; R. G. Brown; and R. G. Record. 1972. "An interpretation of the modern rise of 
population in Europe". Population Studies, vol. 26, no. 3 (November). Interesting exploration of 
possible causes of accelerating population growth in Europe in the eighteenth and nineteenth 

McLellan, A. C, and Boggs M. D.. 1974. Illegal aliens: A story of human misery. ZPG National 
Reporter, December (Reprinted from AFLCIO American federationist, August 1974.) 

Mangin, W. 1967. "Squatter settlements". Scientific American, October. (Scientific American 
offprint 635, W. H. Freeman and Company, San Francisco.) Provides some insight into lives of 
recent migrants into LDC cities. 

Martin, P. S. 1970. "Pleistocene niches for alien animals". BioScience, vol. 20, no. 4, (February 
15) pp. 218-221. Discusses the ecological impact of Pleistocene extinctions. 

and H. E. Wright, eds. 1967. Pleistocene extinctions. Yale University Press, New Haven. On 

the effects of early human hunting on populations of large mammals. 

Morrison, P. A., and J. A. Wheeler, 1976. "Rural renaissance in America? The revival of 
population growth in remote areas". Population bulletin, vol. 31, no. 3. Describes the recent shift 
of the U.S. population toward rural areas. 

Ravenholt, R. T. 1976. "Gaining ground on the population front". War on hunger, February, pp. 
1-3, 13. An optimistic view of current population growth rates and projected trends. 

Razzell, P. E. 1974. An interpretation of the modern rise of population in Europe"-a critique. 
Population Studies, vol. 28, no. 1, pp. 5-17. Critical comment on McKeown, Brown, and Record 
(above) postulating the adoption of soap and sanitation as a major cause of declining death rates 
in eighteenth- and nineteenth-century Europe. 

Reed, C. A. 1970. "Extinction of mammalian megafauna in the Old World late Quaternary". 
BioScience, vol. 20, no. 5 (March 1), pp. 284-288. More on extinctions of populations of large 
mammals due to human activities. 

Sklar, J., and B. Berkov. 1975. "The American birth rate: Evidences of a coming rise". Science, 
vol. 189, pp. 693-700 (August 29). Suggests that United States birth rate will rise in late 1970s as 
women begin bearing postponed children. 

Sweezy, A. 1971. "The economic explanation of fertility changes in the United States". 
Population Studies, vol. 25, no. 2 (July), pp. 255-267. Casts doubt on economic explanations of 
low birth rates during Depression of 1930s. 

Tanton, John H. 1976. "International migration as an obstacle to achieving world stability". The 
ecologist vol. 6, no. 6 (July). On the demographic and social effects of migration today. 

United Nations Statistical Office. Annual. Demographic yearbook. New York. This annual 
compilation is the source for world data on population. 

U.S. Bureau of the Census. 1971. The two-child family and population growth: An international 
view. Washington, D.C., September. Population projections for selected countries. 

U.S. National Center for Health Statistics (USNCHS). Monthly. Vital statistics report. 
Washington, D.C. This report is essential for up-to-date information on vital rates and 
demographic trends in the United States. Other useful publications on the American population 
are also issued from the center. 

Walford, C. 1878. "The famines of the world: Past and present". Royal Statistical Society 
Journal, vol. 41, pp. 433-526. An interesting historical account. 

Waterbolk, H. T. 1968. "Food production in prehistoric Europe". Science, vol. 162, pp. 1093- 
1 102. On beginning of agriculture in Europe. 

Webb, W. P. 1964. The great frontier. Rev. ed. University of Texas Press, Austin. Diverting, 
perhaps prophetic discussion of how the frontier has shaped Western attitudes and institutions 
and how its disappearance may lead to ultimate disaster. 

Wrigley, E. A. 1969. Population and history. McGraw-Hill, New York. Brief and interesting. 

Young, J. Z. \91\.An introduction to the study of man. Clarendon Press, Oxford. Chapter 33 
deals with the history of the genus Homo. The book as a whole is a gold mine of information on 
humanity as seen by one of the world's most distinguished zoologists. 

-246- PAGENUMBER247 


Land, Water, and Forests 

We abuse land because we regard it as a commodity 
belonging to us. When we see land as a community 
to which we belong we may begin to use it with love 
and respect. 

- Aldo Leopold, 1948 

Among physical resources, land is central in importance. By land we mean not only physical 
space but also the characteristics that govern the uses to which land can be put. Those 
characteristics include the topography (the shape of the terrain—for example, flat, hilly, or 
mountainous), the quantity and quality of soil, the availability of water, and the nature of local 
climates. Of course, those features are interrelated, and together they influence and are 
influenced by the vegetation that grows on the land: soil is a product of the underlying rock, the 
climate, the topography, and the creatures living on it and in it; the availability of water depends 
on how much falls as rain and snow, on how much evaporates, and on how much is retained, 
where, and for how long; the latter properties depend on local soil and vegetation as much as 
they influence them. 

Many of the basic physical principles underlying these relations am described in Chapter 2, 
Chapter 3, and Chapter 4. In this chapter we discuss the characteristics of Earth's land and its 
supplies of fresh water as resources—that is, in terms of their availability and suitability for the 
support of the human population. The closely related topics of forests and the production of 
timber are also treated here; production of food is the subject of Chapter 7. 


Earth has a land area of 149 million square kilometers (58 million square miles), which in 1975 
was occupied at an average density of about 27 people per square kilometer 


(70 per square mile). This does not seem to be such a high density—it amounts to almost 4 
hectares (or 10 acres) per person. 

But land is a resource only insofar as its specific characteristics enable it to serve some human 
need. A very rough classification of global land area suggests that only about 30 percent of the 
land surface is potentially arable (farmable); 20 percent is uncultivable mountainous terrain; 20 
percent is desert or steppe; 20 percent is characterized by glaciers, permafrost, and tundra; and 
10 percent consists of other types of land with soils unsuitable for cultivation. - 

Much of the land that is uncultivable is also so inhospitable as to be nearly uninhabitable—the 
Arctic and Antarctic, steep slopes, swamps, certain desert regions, and so on. For good reason, 
then, the human population is spread very unevenly over Earth's land surface. People have 
concentrated— and continue to concentrate today— in the areas that are most hospitable. 

Some of the most serious land problems arise from competing, mutually exclusive uses for the 
same advantageously located pieces of land. Many of our cities, for example, arose in the centers 
of the best farmland, so some of that valuable resource has been lost beneath highways, suburbs, 
and airports as the cities have spread. Coastlines are in demand as desirable places to live, as a 
recreational resource for those who do not live there, as economical sites for electric power 
plants, as outlets for commerce, and as bases for the utilization of marine resources ( Figure 6-1 
). Unfortunately, the coasts are also the location of relatively fragile communities of plants and 
animals, such as those in salt marshes and estuaries, upon which much of the productivity of the 
sea depends (see Chapter 4). Leaving essential ecological systems such as these intact may prove 
to be one of the most important uses of land; it may also prove to be one of the uses that is least 
compatible with other human activities. To assume that human beings should dare to exploit 
every bit of land that appears potentially capable of being exploited would be dangerous 
(although not unprecedented) arrogance. 

In summary, there appears to be a good deal of land available if one does not inquire too 
carefully about what kind it is. But the most useful sorts of land are already in short supply in 
most parts of the world. 

We note also that United States consumers (and those in many other nations that rely heavily on 
imports of food or raw materials) must be considered to be "occupying" a good deal of land 
outside their national boundaries. In this sense, DCs "occupy" coffee plantations in Brazil and 
rubber plantations in Laos, land used for bauxite mines in Jamaica and copper mines in Zambia, 
rangeland for cattle and sheep in Argentina, lumber-producing forests in Ivory Coast and 
Indonesia, and land used for growing soybeans in Colombia and peanuts in Nigeria. 

Classification of Land by Climate, Vegetation, and Use 

Closer examination of Earth's land resource can take several directions: we can classify it 
according to climate and vegetation, according to present use, and according to characteristics of 
soils. In this section of text we consider climate, vegetation, and land use, and we treat soils in 
the next section. 

Vegetation and cultivation. Table 6-1 shows a classification of the land area of the globe into 
bioclimatic regions, carried out by Soviet scientists who have long been active in the field of 
biogeography. The term bioclimatic is merely an explicit recognition that regional climates and 
vegetation patterns are intimately interconnected. An alternative classification strictly by 
vegetation patterns, corresponding very closely in its categories to the biomes described in 
Chapter 4, is given in Table 6-2 . In these classifications, land refers to the area of continents and 
islands, including the lakes and streams they contain. In Table 6-1 the lakes and streams 
constitute a separate category; in Table 6-2 they are apportioned among the biomes in which they 
occur. When land covered by lakes, streams, and glaciers is subtracted from the category of land 
area, the global land surface amounts to 133 million square kilometers. As Table 6-2 indicates, 
about 10 percent of that land surface is under cultivation. 

In 1967 the report of the President's Science Advisory Committee panel on the world food 
supply estimated the 

Georg Borgstrom, Too many, p. 291. 
-248- PAGENUMBER249 

FIGURE 6-1 An example of development on an estuary near San Rafael, California, on San 
Francisco Bay. Houses and boat docks are built right on the shore; a freeway interchange has 
been built over the water. Dikes are shown at the lower left, and even the channels are man- 
made. (Photo by Aero Photographers .) 

TABLE 6-1 

Bioclimatic Regions of the World 

Total land 


Percentage of 



lillion km ) 

total land 

Polar humid and semihumid 



Boreal humid and semihumid 



Subboreal humid 



Subboreal semiarid 



Subboreal arid 



Subtropical humid 



Subtropical semiarid 



Subtropical arid 



Tropical humid 



Tropical semiarid 



Tropical arid 






Streams and lakes 



Note: 100 percent = 149.3 million km . 

Total land area 
Bio climate (million km ) 

Source: L. Rodin, N. Bazilevich, N. Rozov, Productivity of world's main 
ecosystems, in National Academy of Sciences, Productivity of world ecosystems, 
Washington, D.C., 1975, pp. 13-26. 

Percentage of 
total land 

TABLE 6-2 

Land Classification 

by Vegetation 

Type of vegetation 

Total land area 

(millions km ) 

Percentage of 
total land 

Tropical forest 



Coniferous forest 



Deciduous forest 






Semiarid grasslands 



Humid grasslands 






Cultivated (grain) 



Cultivated (other) 









Glacier and perpetual 




Source: After Edward Deevey, The human population. 

TABLE 6-3 

Arable and Cultivated Land and Population, Worldwide 


Land area (million km ) 

area per 

land as % of 


in 1975 





arable land 















Australia and 
New Zealand 














North and 
Central America 







South America 
















area per 

land as % of 




arable land 



Land area (million km ) 

in 1975 Potentially 

Region (millions) Total arable Cultivated 

Total 3967 131.5 31.9 13.89 

Note: Cultivated area is called by FAO "arable land and land under permanent crops." It 
includes land under crops, land temporarily fallow, temporary meadows for mowing or pasture, 
market and kitchen gardens, fruit trees, vines, shrubs, and rubber plantations. Within this 
definition there are said to be wide variations among reporting countries. The land actually 
harvested during any particular year is about one-half to two-thirds of the total cultivated land. 
Populations of some islands omitted. 

Source: For population: 1975 Population Data Sheet, Population Reference Bureau, Inc., 
Washington, D.C. For land: President's Science Advisory Committee, The world food problem; 
Economic Research Service, United States Department of Agriculture, Foreign agricultural 
economic report 298, Government Printing Office, Washington, D.C, 1974. 

amount of potentially arable land on Earth to be 3.19 billion hectares. _This amounts to only 24 
percent of the total ice-free land area but is perhaps three times the area actually planted and 
harvested in any given year. About 1.67 billion hectares—more than half of the estimated total- 
lies in the tropical areas. Warm-temperate and subtropical areas account for another 0.56 billion 
hectares, and cool-temperate areas account for most of the rest (0.91 billion hectares). The 
distribution of cultivated and potentially arable land in relation to population and area of 
continents is shown in Table 6-3 . Most of the land classified as potentially arable but not now 
under cultivation is in Africa and South America. 

It is easy to be misled by the foregoing estimate of potentially arable land into an overoptimistic 
prognosis of how much more land can be brought under cultivation. Several factors make 
optimism unwarranted. First, rather obviously, the best agricultural land is already being 
cultivated. Neither subsistence nor commercial farmers are stupid; they farm first where food can 
be produced with the smallest investments of labor and money (this is the definition of the best 
land). Most land classified as potentially arable but not now under cultivation suffers from one or 
more important specific defects: it may be remote, subject to erosion, steep or uneven, rocky, of 
short growing season, deficient in water, endowed with soils deficient in nutrients, or intractable 
in other respects. (In Africa, for example, tsetse flies make substantial areas uninhabitable; they 
carry sleeping sickness and are very difficult to eradicate.) Land with such deficiencies is called 
marginal, and the difficulty of bringing marginal land into profitable cultivation is the major 
reason the amount of cultivated land has in fact been expanding so slowly (only about 0.15 
percent per year worldwide between 1950 and 1970). - 

Some of the shortcomings of marginal land can of course be overcome by technology. Irrigation 
can supply missing water, fertilizer can supply missing nutrients, terracing can level slopes and 
reduce erosion— but such measures are expensive. The costs of opening new land in seven sample 
projects in LDCs in the early 1960s ranged from $87 to $2400 per hectare, and the median was 
$540. ^_If the very optimistic assumption is made that in 

President's Science Advisory Committee, The world food problem. 
University of California Food Task Force, A hungry world: the challenge to agriculture. 
President's Science Advisory Committee, The world food problem, vol. 2, pp. 435-439. In 
1974 the United Nations estimated that the total cost of a 20-percent expansion of land under 
cultivation, renovation of existing irrigated areas, and development of new irrigation schemes 
would cost $90 billion. For potential gains to be exploited rapidly enough, an annual 
investment of between $8 billion and $8.5 billion would be required. ( Assessment of the 
world food situation, present and future, item 8, provisional agenda, U.N. World Food 
Conference 1974). Since the United Nations total corresponds to a cost of only $300 per 
hectare of newly cultivated land, it would appear to be a substantial underestimate. 
-250- PAGENUMBER251 

1975 land suitable to feed four persons per hectare could be brought under cultivation for 
$1000/hectare, then an investment of $20 billion annually would be required to keep up with 
present world population growth of about 75 million per year. Of course, the greater the amount 
of marginal land brought under cultivation, the lower will be the quality of the remaining unused 
pool and the higher will be the economic costs of further increments to cultivated land. 

Another basis for skepticism about the potential for expanding cultivated land is that much of the 
land classified as potentially arable is already in use as pasture or economically productive 
forest. Grazing land makes up perhaps two-thirds of the presently uncultivated but potentially 
arable land worldwide, and accessible forest about one-third. ^Putting grazing land under the 
plow delivers a smaller net yield than opening virgin land, since the former is already producing 
some food. Clearing forest for cultivation increases food production at the expense of actual or 
potential production of timber, a versatile raw material also subject to increasing demands. (The 
ecological consequences of changes in land-use patterns are discussed in Chapter 1 1 .) 

Land use in the United States. With a land area of 9.16 million square kilometers (about 3.5 
million square miles) and a population in 1975 of around 215 million people, the United States 
has an average population density slightly less than that of the world as a whole-with about 24 
people per square kilometer, or 62 per square mile. ^_As noted in Chapter 5, most of these people 
are concentrated in urban regions on the coasts and around the Great Lakes, leaving large 
segments of the central United States sparsely populated (by the superficial measure of land area 
per person). The land area of the forty-eight contiguous states (which for some purposes is more 
relevant than the area including Alaska and Hawaii) is 7.69 million square kilometers, and the 
corresponding population density is 28 people per square kilometer-about the same as the global 

TABLE 6-4 

Ownership of Land in the United States (including Alaska and Hawaii) 

Area Percentage 

Owner (million hectares) of total 

Federal government 

Department of Interior 219 



(million hectares) 

of total 















Department of Agriculture 
Department of Defense 

Total federal 
State governments 
County and municipal 


Indian tribes 

Individuals and corporations 

Total 916 100.0 

Source: United States Department of Commerce, Statistical abstract 
of the United States, 1974, p. 199. 

in Table 6-4 . The holdings of the federal government amount to more than a third of the total. 
Most of the federal land is grazing land and forest, controlled by the Bureau of Land 
Management in the Department of the Interior and by the United States Forest Service in the 
Department of Agriculture. Land in private ownership is about 60 percent of the total, while the 
original occupants of the territory now comprising the United States—tribes of American Indians- 
-control about 2 percent. If Alaska is excluded, the fraction of territory held by the federal 
government falls to about 20 percent, Indian land is 3 percent, and the fraction in private hands 
rises to about 70 percent. 1 

The uses of land in the United States around 1970 are summarized in Table 6-5 . The category 
labeled "cropland" represents the sum of cropland actually in use as well as that which is 
deliberately held idle for economic or political reasons. Some 12 to 14 percent of the cropland 
total was idle in the early 1970s, but essentially all of it had been pressed into service by 1975. - 
Urban and transportation uses of United States territory appear 

Borgstrom, Too many, p. 299. 
6 The area given is without lakes and rivers; with them, the figure is 9.37 million square 

kilometers. ( United States Department of Commerce, Statistical abstract, 1975), p. 5. 
7 C. B. Hunt, Physiography of the United States ( W. H. Freeman and Company, San Francisco, 

1967), p. 121. 

Lester R. Brown, The world food prospect; see also Chapter 7. 
-251- PAGENUMBER252 

TABLE 6-5 

Uses of Land in the United States 




(million hectares) 

of total 


















Pasture and rangeland 

Ungrazed forest 


Desert, swamp, barren, 

tundra (limited use) 
Urban and transportation 

National wildlife refuges 
National parks 

Farm buildings and 
farm roads 11 1 .2 

Withdrawn from other uses 

by surface mining 2 0.2 

Transmission line rights 
of way 1 0.1 

Other 24 2.6 

Note: The table, which includes Alaska and Hawaii, does not include 
lakes and reservoirs. 

Sources: United States Department of Commerce, Statistical abstract 

of the United States , 1974, p. 600; National Commission on Materials Policy, 

Material needs and the environment today and tomorrow, 

chapter 7. 

rather minor, as they involve less than 3 percent of the total land area, but it should be noted that 
some expanding urban centers are located in the midst of or adjacent to the best agricultural 
lands. The nation can ill afford to lose those prime croplands. California, where the problem of 
such encroachment has been particularly severe, had lost about 700,000 hectares of prime 
agricultural land to cities, suburbs, and highways by 1975. And of California's 8.2 million 
hectares of remaining prime and "potentially prime" agricultural land, another 400,000 hectares 
were zoned for urban development by 1985. ^_The loss of arable land to urbanization nationwide 
has been estimated at 5.1 million hectares between 1958 and 1974. — 


The nature and quantity of vegetation that can be grown on a given piece of land depend strongly 
on the characteristics of the soil. Study of the properties and distribution of soil types is therefore 
essential both for an understanding of existing patterns of vegetation and for determining 
strategies and prospects for success in increasing the yield of plant materials used by human 

Soil functions as a source and as a storage reservoir of water and mineral nutrients for plants, as 
a medium in which chemical transformations influencing the availability of those nutrients takes 
place, and as an anchor for the support of terrestrial as well as many aquatic plants. In 
composition, it is a mixture of inorganic minerals, organic matter, soil animals and 
microorganisms, gases, and, of course, moisture. — 

Soil origins and structure. As described in Chapter 2, the main source of the inorganic particles 
in soil is the weathering of the various types of rock—igneous, sedimentary, metamorphic— by 
exposure near Earth's surface to temperature changes; the physical action of ice, water, and roots; 
and chemical attack by rainwater, surface water, and atmospheric gases. Uplifting of 
unconsolidated sediments from the seafloor is another, rarer source of soil materials. These 
sources of the mineral components of soil are called parent materials; the parent materials can be 
either residual (meaning the soil is formed where the weathering takes place) or transported 
(meaning the materials have been moved to the place of soil formation by the action of wind, 
water, ice, gravity, or a combination of these). Alluvial is the term given to soils deposited by 
river flow. Colluvial refers to soils whose parent materials were deposited by landslides. 

Most soils are not uniform in vertical profile but rather are characterized by zones called 
horizons. The uppermost layer or zone is called the A horizon. It is the home of most of the soil 
organisms and the location of the greatest abundance of roots. It has usually been somewhat 
depleted of soluble substances by leaching. The next zone proceeding downward is the B 
horizon. It receives downward-moving minerals leached from above and, often, upward- 
migrating substances from weathered parent materials below; therefore it is called the zone of 
accumulation. The C horizon consists of the weathered rock material. The bedrock below, 
usually but not always the true parent material, is the D horizon. 

10 University of California Food Task Force, A hungry world p. 78. 

n The best introductory consideration of soil we have found (on which much of this section is 
based) is J. Janick, R. Schery, F. Woods, and V. Ruttan , Plant science, chapters 12 and 16. 
California Land Use Task Force, The California land, Chapter 3. 
-252- PAGENUMBER253 

In soil studies more detailed than our treatment here, the horizons are often subdivided by the 
addition of subscripts running 1 to 3, top to bottom, (that is, A i, A 2, A 3 ). In that notation, the 
fresh dead organic matter (called litter) is the A 00 horizon; and the partially decomposed organic 
matter (called duff), just above the A 1 horizon, is the A horizon. 

Soils in which the layered structure is well developed and distinct are called zoned or normal; 
those without this well-developed vertical profile are called azonal. Alluvial and colluvial soils 
are often azonal. 

Another important characteristic of soil is texture (meaning the size of the individual mineral 
particles). An international classification system defines particles with diameter less than 0.002 
millimeters as clay, those with diameters between 0.002 and 0.02 millimeters as silt, and 
particles between 0.02 and 2 millimeters in diameter as sand. The term loam is used to 

describe mixtures of the different size classes of soil particles. A standard classification scheme 
based on the proportions of clay, silt, and sand in soil is illustrated in Figure 6-2 . 

Soil texture influences the rate at which water percolates through soil and the amount of water a 
soil can contain. Coarse soils are characterized by rapid infiltration, and hence low surface 
runoff, but they cannot retain much water. The fine-textured clays are penetrated by water only 
slowly but have a high storage capacity. The pore space in soil, which is filled in varying 
proportions by water and air, is typically around 50 percent in many kinds of soils; what is more 
important than the total volume of the pore space is the characteristic size of individual pore 
spaces. Few large pores make a much less satisfactory soil than many small ones. Soil organisms 
and organic matter are crucial in maintaining the better situation by preventing excessive 
coagulation of soil particles into large clods. 

Soil organic matter. Dead organic matter in soil not only influences pore size, but itself serves 
as a sponge that soaks up and retains moisture. Through its decomposition by bacteria, it acts 
also as a source of carbon dioxide, water, and mineral elements. Certain constituents of 

atjan InrDrrr i-ton: EcokTCe: Ecosz« - c= 

s. &nvTtremtnt. Lontrtoutors: Raul PL Hiric^ - BLTtwr, i 
= _: -2: "* =£- =-=" zz- =.r caTcr. Y±2 _ : 13 
on of M- use, may riat b= fMllH I crpferi. 1&L1I 

Sard {%) 

FIGURE 6-2 Soil classification by texture. (From Janick et al. Data from U.S. Department of 


dead organic matter, such as waxes, fats, lignins, and some proteinaceous materials, resist 
decomposition and are converted instead into the dark colloidal substance called humus. (A 
colloidal substance consists of particles larger than molecules but small enough to remain 

suspended in solution.) The physical and chemical properties of humus affect the character of 
soil out of proportion to its fraction by weight, in part because of the large surface-to-volume 
ratio associated with particles of such small size. The fraction by weight of dead organic matter 

1 3 

in soil is typically in the range of 0.4 to 1.1 percent. — 

Living soil organisms (other than the roots of plants) make up an even smaller fraction of the 
mass of soil (typically 0.1 percent or less). ^_(In absolute terms this is not a small quantity, 
however, as it amounts to several tons of living organisms per hectare.) These include: bacteria 
and algae, whose critical roles in nutrient cycles are described in Chapter 3; fungi, which 
perform some of the same functions as bacteria in nutrient cycles, as well 

1 9 

See Janick et al., Plant science, p. 219, for a more complete classification, ranging up to 
"boulder" (diameter greater than 256 millimeters). 

1 3 

P. Sanchez and S. Buol, Soils of the tropics and the world food crisis. 
Janick et al., Plant science, p. 224. 
-253- PAGENUMBER254 

as being the principal producers of humus; and larger organisms such as mites, millipedes, 
insects, and worms, which physically cultivate the soil and help break down organic litter (see 
also Chapter 11). 

Soil chemistry. The chemistry of a soil is governed in large measure by the properties of its clay 
particles and the similarly tiny particles of humus. Clay particles are platelike, with a layered 
internal structure that leaves negative electric charges arrayed on the surfaces of the plates. 
Humus particles are also negatively charged on their exteriors. These electrical properties 
account for the characteristic of soil exchange capacity (the ability to retain and exchange cations 
such as H + , Ca ++ , Mg ++ , K + , and Na + ). 

This function is crucial in governing soil fertility. Without the negative charges on particles of 
clay and humus, the positively charged nutrient cations released to the soil by the decay of 
organic matter or added to the soil in fertilizer would quickly be leached away, out of reach of 
plant roots. Bound to the negative charges on clay and humus, however, those ions are made 
available to plants gradually when they are replaced at the negatively charged sites by hydrogen 
ions from the soil. There is actually a replacement heirarchy—hydrogen, calcium, magnesium, 
potassium, sodium—in which each ion listed tends to displace any that appears to its right in the 
list, if the two are present in equal amount; hydrogen tends to replace all of the metallic ions. If a 
particular cation is present in very large quantities, however, it can even replace ions to its left in 
the heirarchy by sheer force of numbers (that is, by mass action, to use chemical terminology). 

The magnitude of exchange capacity is merely a matter of how many negatively charged sites 
are available in a soil. The unit of measure is milliequivalents per 100 grams, which means the 
number of milligrams of hydrogen ion (H+) that will combine with 100 grams of dry soil. 
Different types of clay have widely varying exchange capacities (in the range of from 10 to 100 
milliequivalents/ 100 g), whereas humus has the greatest of all (from 150 to 300 
milliequivalents/ 100 g). 1S This chemical role of humus is perhaps even more crucial than the 

roles humus plays in maintaining soil texture and retaining water. The importance of all three 
functions provides ample reason for viewing with alarm the depletion of soil humus by certain 
agricultural practices (see Chapter 11). 

A major source of hydrogen ions in the soil is the production of carbonic acid from the solution 
of carbon dioxide in water. Another source is nitric and sulfuric acid added by polluted rainfall 
or formed from nitrogen and sulfur compounds produced by decomposition of organic matter or 
added in fertilizer. Yet another is organic acids exuded by plant roots or produced by 
decomposition. Excessive concentrations of hydrogen ions (indicated by low pH~that is, 
strongly acidic soil) replace nutrient cations on clay and humus colloids faster than those 
nutrients can be taken up by plants; the result is that the nutrients are leached away. Acidic soils 
are common in humid climates. A shortage of hydrogen ions (high pH~strongly alkaline soil), on 
the other hand, can leave some nutrient ions too tightly bound to clay and humus to be absorbed 
by plants. Other nutrients, such as iron and manganese, become trapped in compounds that are 
extremely insoluble in basic solutions. 

Crops differ rather widely in the pH ranges they tolerate. Most of the grains flourish best in 
slightly acidic soil, potatoes and berries in quite acidic soil, alfalfa and asparagus in neutral soil. 
_^_When soil is too acidic for the crops desired, it is common practice to neutralize the soil by 
adding lime. 

Classification of soils. Differences in soils around the world arise from the mineral compositions 
of the parent materials and from differing climatic conditions, which together influence the 
organic and inorganic processes of soil development. Several major types of soil-forming 
regimes have been identified: the main ones are podzolization, laterization, calcification, 
gleization, and salinization. 

Podzolization is the set of processes associated in its extreme form with cool climates, abundant 
precipitation, and acid upper soil layers strongly leached of mineral nutrients and the oxides of 
iron and aluminum. The nutrients, oxides, and humus accumulate in the deeper 


Janick et al., Plant science, p. 222, see also A. N. Strahler and A. H. Strahler , Environmental 
geoscience, Chapter 1 1 . 
Janick et al., Plant science, p. 309. 





gr« an) <Kt daiart 

fM . ', ..".I 

FIGURE 6-3 Relation of soil-forming regimes, vegetation, and soil types. (Adapted from Janick 

et al, 1974.) 

layers. Fungi are the main soil-forming organisms. Those soils are characteristic of northern 
forests, although they exist in some circumstances well into temperate and subtropical regions. 

Laterization is a set of processes associated with the humid tropics and subtropics. High mean 
temperatures in these regions permit sustained and rapid bacterial action, which minimizes the 
accumulation of plant litter and humus. In the absence of the organic acids associated with 
humus, the soil is neutral, rendering the oxides of iron and aluminum relatively insoluble; those 
oxides accumulate in the upper soil horizons as hard clays and rocklike material called laterite 
(from the Latin for brick), a mixture of Fe 2 O 3 • nH 2 O and Al 2 O 3, • H 2 O. 

Calcification occurs in climates where evapotranspiration exceeds precipitation. There is little 
leaching of the metallic cations, and microbial activity is slow, so the soils tend to be alkaline 
and rich in humus. Calcium carbonate in solution is carried upward from the water table by 
capillary action in the season of little surface moisture and is left in the upper soil horizons in 
solid form when the water evaporates. 

Gleization is the set of processes characteristic of poorly drained environments in cool or cold 
climates. The low temperatures permit heavy accumulation of organic matter, and the excessive 
wetting produces a sticky clay underneath. 

Salinization is the regime characterized by accumulation of highly soluble salts in the soil. This 
situation arises naturally from poor drainage in regions of low precipitation and high 
temperatures (deserts), and it can be brought about by faulty agricultural practices-irrigating too 
parsimoniously in dry climates, or using salt-laden irrigation water. Soils in the salinization 

1 n 

regime are weakly to strongly alkaline. — 

The geographical boundaries separating the spheres of influence of the different soil-forming 
regimes are often not distinct. An enormous variety of soils results from variations within 
regimes and from overlaps between regimes, and a quite complicated classification scheme and 
taxonomy for soils (the United States comprehensive system of soil classification) has been 

1 8 

devised to help soil scientists cope with this diversity. Figure 6-3 is a schematic illustration of 
how a few of the principal soil types emerge from the soil-forming regimes. Characteristics of a 
somewhat broader selection of soil types encountered in the literature of soil science and 
biogeography are summarized in Box 6-1. — 

A somewhat more utilitarian soil classification scheme, widely used in the United States, is 
based on the capability of a soil to sustain various uses—the categories run from Class I (the best 
soils for agriculture) to Class VIII (suitable only for recreation and wildlife). — 

Exhaustion, erosion, and regeneration. The limitations in the extent and quality of the soils 
with which civilization began would be serious enough, 

1 7 

For more extensive treatments of these regimes, see Strahler and Strahler, Environmental 
chapter 12; Janick et al., Plant science, chapter 12; C. B. Hunt, Physiography, chapter 6. 

1 8 

See Janick et al., Plant science, pp. 232-235, for a summary, or United States Soil 
Conservation Service, Soil classification: A comprehensive system ( Government Printing 
Office, Washington, D.C., 1964) for the full taxonomy. 

See also H. W. Menard, Geology, resources, and society, chapter 13. 

Janick et al., Plant science, pp. 234-236. 

Tundra soil 

Alpine meadow 

BOX 6-1 Characteristics of Common Soils 

Pedalfers (soils of humid regions) 

Dark-brown, peaty layer over gray horizons mottled with rust; 

permanently frozen. Climate: frigid, humid. Vegetation: 

mosses, herbs, shrubs. Process: gleization (development of 
organic-rich, sticky, compact, clayey layer due to excessive 


Dark-brown, organic-rich layer grading down at 30 to 60 
centimeters to 

gray and rusty soil, streaked and mottled. Climate: cool 
temperate to 

frigid. Vegetation: grasses, sedges, herbs. Process: gleization, 

calcification (deposition of calcium carbonate). 

Brown, dark-brown, or black peaty material over soils of 

matter mottled gray and rust. Climate: cool to tropical; 

humid. Vegetation: swamp, forest, sedges, or grasses. Process: 


Leaf litter over a humus-rich layer over a whitish-gray to 
grayish-brown leached layer; B horizon clayey and brown. 

Climate: cool, temperate, humid. Vegetation: northern forests, 
coniferous and/or deciduous. Process: podzolization (bases- 

Al and 
Fe~leached more than silica from A horizon and accumulated 


Thin, dark organic layer at surface over yellow-gray or gray- 

leached layer 15 to 90 centimeters thick, over clayey B horizon 

parent material mottled red, yellow, and gray. Climate: warm 

Bog and half-bog 

Podzol, brown podzol, gray- 

Red and yellow podzol 

Prairie and reddish prairie 


Chestnut, brown, reddish 
and reddish brown 

Desert, sierozem, red desert 



Pedalfers (soils of humid regions) 

temperate to tropical humid. Vegetation: coniferous forest or 

deciduous and coniferous. Process: podzolization 
superimposed on 

lateritization (silica leached more than the bases). 

Brown in the north; reddish-brown toward the south. Grades 

down to 
lighter-colored parent material with no horizon of carbonate 
accumulation. Climate: cool temperate to warm temperate, 

Vegetation: tall grass. Process: weak podzolization. 

Pedocals (soils of arid regions) 

Black to gray-brown, crumbly soil to a depth of 90 to 120 

grading through lighter color to a layer of carbonate 

Climate: subhumid, temperate to cool. Vegetation: tall grass. 

calcification (accumulation of carbonates in lower horizons). 

Brown to black surface layer in north; reddish in south; lighter 
color at 

depth and grading down to layer of carbonate accumulation. 

Chestnut thinner than chernozem, and brown thinner than 

Climate: semiarid; cool to hot. Vegetation: mostly short 
grasses in 

north; grasses and shrub in south. Process: calcification. 

Light gray or brown in north; reddish in south; low in organic 

carbonate layer generally within 30 centimeters of the surface. 

Climate: arid, cool to hot. Vegetation: mostly desert shrubs. 


Other soils 

Dark-gray or black, organic-rich, surface layers over soft light 
gray or 

white calcareous material derived from chalk, soft limestone, 
or marl. 

Climate: variable. Vegetation: mostly grassland. Process: the 
lime in 

these soils is derived from the parent materials. 

Thin organic layer over reddish, strongly leached soil, generally 

Pedalfers (soils of humid regions) 

and enriched in hydrous alumina or iron oxide or both; low in 

generally many feet thick. Climate: tropical wet. Vegetation: 

forest. Process: lateritization (see Red and Yellow Podzol). 

Soils in which salts including alkali have accumulated, generally 


_, ,. , ,, ,• poorly drained areas. Climate: variable, but commonly arid or 

Saline and alkaline r . J . , T . 1x A . . , , . 

semiand. Vegetation: salt-tolerant species or lacking. Process: 

salinization or alkalization (salts deposited in soil as a result of 


Source: Hunt, Physiography of the United States. 

-256- PAGENUMBER257 

considering the demands that a growing population is placing on this resource. Those limitations 
are aggravated, however, by the loss of needed soil by urbanization, by erosion, and by the 
depletion or exhaustion of the fertility of some soils. 

Depletion of soil nutrients and humus need not follow from intensive cropping, as demonstrated 
by the continued fertility of rice paddies in Asia that have been cultivated continuously for 
thousands of years. That success has been due to several factors: the high clay content of the soil, 
the annual augmentation of the alluvial topsoil in the paddies by silt eroded from hilltops, the 
nitrogen fixation by blue-green algae in the flooded paddies, and the practice of returning human 

9 1 

and animal waste to the soils from which the nutrients therein came. Intensive cultivation 

without either natural or artificial augmentation of nutrients can only exhaust soil fertility, 
however, and even the use of inorganic fertilizers alone does not prevent the depletion of the 

humus (see also Chapter 11). The practical application of the detailed study of soils is to 

discover both the potential and the limitations of different soil types for the production of 
vegetation, and the procedures appropriate to using each type as a permanent or renewable 
resource rather than as a "mine" to be exhausted and abandoned. 

Perhaps even more serious than urbanization and loss of fertility as a global threat to soils is 
erosion (the removal of the body of the soil itself by the action of wind and, especially, water). 
Cultivation of soil increases the natural rate of erosion, even when it is conducted properly on 
good land. Data from the United States Department of Agriculture indicate that erosion rates on 

cultivated land may be 100 times those on forested land receiving the same rainfall. One of the 

most careful analyses of erosion in the United States and worldwide suggests that the transport of 
sediment from the land to the sea has increased more than threefold in the United States over the 

prehuman rate and by a factor of about 2.5 worldwide. (The study estimated the sediment 

loads in the 1960s as 180 metric tons per year per average square kilometer of ice-free land 
globally and 250 metric tons per square kilometer per year in the United States.) The map in 
Figure 6-4 indicates the cumulative toll erosion has taken in the forty-eight contiguous states. 

Loss of soil by erosion is a significant problem for the United States and other industrial nations, 
but it is worse by far in the poor countries. There even the modest degree of control exerted in 
the United States by the Soil Conservation Service is often missing, and the pressure of rapid 
population growth makes itself felt in overgrazing, deforestation for firewood, and the clearing 
of steep slopes for cultivation. The result has been an appalling increase in rates of erosion both 
by wind (primarily in desert lands) and by water, precisely in those parts of the world that can 


least afford the loss. We return to the intertwined problems of cultivation practices, climate, 

and the loss of soil or soil fertility in Chapter 1 1 . 

The rate at which lost soil can be regenerated by natural processes varies enormously, depending 
on climate, other factors influencing biological activity, and availability of inorganic raw 
materials. Under very favorable circumstances, in which abundant new material is provided in 
the form of windborne or waterborne sediment or volcanic ash, a foot of soil may form in as little 
as from 50 to 100 years. In the more common situation, in which new soil must be formed 
from parent rocks, the time scale is more likely to be in the range of from 200 to 1000 years per 


centimeter of soil — perhaps 10,000 years or more for a foot of soil. " In most circumstances, 
then, the loss of soil is irreversible on a time scale of practical human interest. 


"Water is the best of all things," said the Greek poet Pindar. It is also, in the broad sense, a 
renewable resource, continually reprocessed and delivered to the land by the hydrologic cycle 
described in Chapter 2. 

The central role of water in sustaining the life processes of the biosphere has been discussed in 

21 Manard, Geology, pp. 361-362 

W. A. Albrecht, Physical, chemical, and biochemical changes in the soil community. 
23 Menard, Geology, pp. 359. 


S. Judson, Erosion of the land. 

Erik P. Eckholm has documented this much-underrated aspect of the global predicament in a 
brilliant book, Losing ground. 
Menard, Geology, pp. 364-365. 
Strahler and Strahlei 


77 - — 

Strahler and Strahler, Environmental, p. 275. 


■SlltfitOf TMWHT 

24 to "H pwwnl c< topt«J tori, may tojvp Wrti» 

**« V__| 

Mont 1hm 75 JWEWf 01 top»l «HL m*j l*t *i*t*0u» V 
a— p gj Hm mduiM* Hv*t« gwlogicaJ wwon n tun* 

of low r-amfiR itmb 

Utty vnal iivw couo "ch t» ihuwr, w rta Kjy» 

Cumulative effects of erosion in the United States. (Adapted from Hunt, 1974.) 

3, and, from a different point of view, in the foregoing section of text on soil. Here we take up 
the availability and functions of water as it is used directly as a resource by civilization: for 
changing the natural pattern of biological productivity through irrigation; for drinking, cooking, 
washing, and bathing; as an industrial raw material and coolant; as a flow resource for diluting 
and removing waste materials, turning hydroelectric generators, and providing transportation and 

We survey here the availability of water for these purposes, present and projected patterns of 
water use and supply, and the nature and costs of large-scale watersupply technology. Pollution 
of water supplies by the activities of civilization is taken up in Chapter 10. 


As we discussed in Chapter 2, the amount of water available for runoff on the surface and 
underground (including not only what reaches the oceans but also net recharge, if any, of surface 
reservoirs and underground aquifers) is simply the difference between what falls as precipitation 
and what returns to the atmosphere as evapotranspiration. The annual runoff from major regions 
of the world is summarized in Table 6-6 . (There is a range of about ± 25 percent in the figures 
published by various scholars; the estimates in Table 6-6 fall at the high end of the range.) The 
runoff of the forty-eight contiguous states is about 1700 cubic kilometers per year, 


FIGURE 6-5 Flow of rivers in the United States (millions of acre-feet per year). (Adapted from 

Hunt, 1974.) 
-259- PAGENUMBER260 

TABLE 6-6 

Freshwater Runoff in Major Regions of the World 

Annual runoff 


(km ) 

i acre feet, 
\ millions 










Europe - 



North and Central America 



South America 






*Excluding USSR. 

Source: Calculated from Study 

of Man's Impact on 

Climate, Inadvertent climate modification 


97; and 

I. P. Gerasimov, D. L. Armand, 

and K. M 

. Yefron, eds., 

Natural resources of the Soviet Union, p. 




T8S0 1890 1900 19X0 19?0 193Glf 1940 1950 i960 19?0 


d JU 





i i i i x v^ i i 


ldBQ 1890 1900 1910 1920 1930 1940 1950 1960 1970 
FIGURE 6-6 Historical variation in mean discharge of two American rivers. Data are five-year 
running averages, plotted at one-year intervals, of mean discharge rate in thousands of cubic feet 
per second (cfs). One thousand cfs = 2.45 million cubic meters per day = 723,000 acre-feet per 

year. (Adapted from Leopold, 1974.) 

only a modest fraction of the total for North America and Central America together. Including 


Alaska and Hawaii, the United States runoff is about 2000 cubic kilometers per year. — 

Dividing the runoff figures by the human population gives what appears at first glance to be a 
reassuring picture. For example, the runoff for the forty-eight contiguous states works out to 
about 4700 billion liters per day, or roughly 22,000 liters per inhabitant per day. The global 
figure is around 31,000 liters per person per day, based on a population of 4 billion. 

Geographical and temporal distribution. These often-cited figures are highly misleading, 
however, because the water is very unevenly distributed, both geographically and in time. 
Worldwide, much of the runoff is in sparsely populated regions, as exemplified by the vast 
drainages of northern Canada (the Mackenzie and other rivers), Siberia (the Ob, Yenisey, and 
Lena), and the jungle of the Amazon. In the United States, the western half of the land area gets 
only about a third of the runoff. (Flows of major United States rivers appear in Figure 6-5 .) 

Even more troublesome than the inconvenient geographical distribution of runoff is its uneven 
distribution in time. Floods and droughts, after all, are age-old problems with water supply. 

Difficulties are caused both by the generally predictable annual cycle of runoff, wherein much of 
the total flow is concentrated in a few months of wet season or spring melt, and by unpredictable 
year-to-year variations in the exact timing of this pattern and in the total annual flow. The 
fraction of the total runoff concentrated in a flood period lasting two or three months is from 40 

to 70 percent over much of the United States. The problem of variation in total annual flow 

from year to year in two of America's major rivers is illustrated in Figure 6-6 . 

A large variation in runoff from month to month and from day to day means that the minimum 
daily flow that 


For various runoff estimates, see Menard, Geology, chapter 17; Strahler and Strahler, 
Environmental chapter 13; Hunt, Physiography, chapters 5 and 9; T. Van Hylckama, Water 
resources. One cubic kilometer (a billion cubic meters) is 264 billion gallons or 810,000 acre- 

C. Murray and E. Reeves, Estimated use of water in the United States in 1970. 
Hunt, Physiography, p. 75. 

Box 6-2 Availability and Dependability: How Much Water Can One Count On? 

Several indicators are in widespread use that represent how much water is usually available or 
how much can be made available in theory. (Considerable confusion has been engendered by the 
inconsistent use of terminology and definitions for these measures in the literature on water.) For 
example, the United States Geological Survey presents data for the annual flow exceeded 9 years 
out of 10. This figure amounts to about 1000 cubic kilometers for the forty-eight contiguous 
states as a whole (about 60 percent of the average annual flow), but in some drainage basins 
within the forty-eight states (those of great variability) the figure is less than a third of the 
average flow. _(It may be useful to ponder the difference between average flow and flow 
exceeded 9 years out of 10 for the individual rivers in Figure 6-6 .) 

A different indicator is dependable flow, which according to the water experts at Resources for 
the Future (a respected private think tank in Washington, D.C., which deals with resource 

questions) means the flow that is equaled or exceeded every day of every year. Because of 

seasonal and year-to-year variations, this would be a very small fraction of annual flow indeed, 
without society's intervention in the form of dams to store water and regulate flows. As it is, 
figures for dependable flow by this definition are rarely encountered. One finds, instead, figures 
for the flow available 50 percent of the time or 90 percent of the time or 95 percent of the time 
(that is, flow rates equaled or exceeded 50 out of 100 days, and so forth). For example, the flow 

available 50 percent of the time for the 48 contiguous states, with the storage facilities of the 
mid-1950s, was 2.2 billion cubic meters per day — less than half of the daily average obtained by 
dividing the average annual flow by 365. _The flow available 95 percent of the time, according 
to the same analysis, was only 0.35 billion cubic meters per day, or about 7 percent of the 
average runoff. (The reservoir storage associated with these availabilities was 220 cubic 
kilometers, or 280 million acre-feet.) As noted in the text, the storage system and available flows 
had been much improved by 1970. 

Finally, Resources for the Future defines maximum dependable flow as the average daily flow 
(the annual flow divided by 365) minus the evaporation losses that would occur in a system of 
reservoirs large enough to spread the flow out completely evenly over a year. If dependable 
supply is defined as above, this is literally the largest value it could have. Recent estimates of the 
maximum dependable flow for the fortyeight states are around 3.6 million cubic meters per day, 
or 80 percent of the average daily flow. In other words, evaporation losses from a system of 
reservoirs extensive enough to parcel out United States runoff uniformly over the year would 
amount to 20 percent of the runoff. 

can be counted on year-round — or even most of the time — is much smaller than the average 
daily flow calculated by dividing the yearly figure by 365 (see Box 6-2). To even out the flow by 
catching floodwaters and releasing them during dry spells, civilization has strung dams along 
most of the world's major rivers. As the network of dams grows more extensive and the 
associated reservoirs grow larger, the dependable flow becomes a bigger fraction of the average 
flow. The flow available 98 percent of the time (98 days out of 100) in the forty-eight contiguous 
states in 1970 was about 1.5 billion cubic meters per day (about a third of the average flow). — 

It is important to recognize that increases in available flow achieved by means of reservoirs are 
bought at the expense of increased loss by evaporation, because of the larger surface area of the 
reservoirs. In other words, regularity increases; total flow decreases. This problem of evaporation 
is especially severe in the southwestern United States and other and regions. The evaporative 
loss at Lake Mead on the Colorado River, alone, was found to be 1 cubic kilometer per year 
(about 4500 liters for every person in the United States), ^_and losses behind Egypt's Aswan 
High Dam have far exceeded initial estimates. 

C. Murray and E. Reeves, Estimated use of water in the United States. 
H. Landsberg L. Fishman, and J. Fisher, Resources in America's future, p. 379. 
R. G. Ridker, Future water needs and supplies. 
J. Hirshleifer, J. DeHaven, and J. Milliman, Water supply, p. 16. 
landsberg, Fischman, and Fisher, Resources, p. 380. 
-261- PAGENUMBER262 

BOX 6-3 Sources and Effects of Dissolved Constituents and Physical Properties of Natural 

Constituent or 
physical property Source or cause Significance 

Silica (SiO 2 ) Dissolved from practically all Forms hard scale in pipes and boilers 

Constituent or 
physical property 

Iron (Fe) 

Calcium (Ca) and 
magnesium (Mg) 

Sodium (Na) and 
potassium (K) 

Bicarbonate (HCO 3 ) 
carbonate (CO 3 ) 

Source or cause 

and soils, usually 1 to 30 ppm 

Dissolved from most rocks and 
soils; also derived from iron 
pipes. More than 1 or 2 ppm 

soluble iron in surface water 
usually indicates acid wastes 
from mine drainage or other 

Dissolved from most soils and 
rocks, but especially from 
limestone, dolomite, and 

Dissolved from most rocks and 

Action of carbon dioxide in 
on carbonate rocks. 


and on 
blades of steam turbines. 

On exposure to air, iron in groundwater 

oxidizes to reddish-brown sediment. 

than about 0.3 ppm stains laundry and 

utensils. Objectionable for food 

Federal drinking water standards state 

iron and manganese together should 

exceed 0.3 ppm. Larger quantities 

unpleasant taste and favor growth of 


Cause most of the hardness and scale- 

properties of water. Waters low in 

and magnesium are desired in 

electroplating, tanning, dyeing, and 


Large amounts, in combination with 

give a salty taste. Sodium salts may 

foaming in steam boilers, and a high 

sodium ratio may limit the use of water 


Produce alkalinity. Bicarbonates of 

and magnesium decompose in steam 

and hot-water facilities to form scale 

release corrosive carbon dioxide gas. In 

combination with calcium and 

cause carbonate hardness. 

Constituent or 
physical property 

Sulfate (SO 4 ) 

Source or cause 

Dissolved from many rocks 


Sulfate in water containing calcium 

hard scale in steam boilers. Federal 
drinking water standards recommend 

the sulfate content not exceed 250 ppm. 

Building a system of water storage and regulation that can deal with year-to-year variations in 
total runoff is vastly more difficult than evening out the peaks and valleys within a single year, 
because a much larger amount of water would have to be stored. Even determining the average 
annual flow can be a difficult and precarious business, as the famous case of the Colorado River 
Compact illustrates. The compact, signed in 1922, divided up the rights to Colorado River water 
among seven contending states ( Mexico was added later), based on the supposition that the 
average annual flow at Lees Ferry would be 22.8 billion cubic meters. (The treaty allocated 
absolute amounts of water, not percentages.) The estimate of the available flow was based on the 
record before 1922. Unfortunately for the effectiveness of the compact, a long dry spell was then 
entered; the average flow at Lees Ferry was 16 billion cubic meters per year in the period 
between 1930 and 1964, and it may well not return in this century to the value assumed by the 
compact in 1922. 


Groundwater. Much cheaper than building reservoirs to even out the variations of surface 
runoff is the time-honored technique of falling back on natural underground water storage — 
groundwater ~ in times of low surface flows. Reservoirs of groundwater (aquifers) are recharged 
naturally by seepage from the surface and underground flows at rates that depend on the 
permeability of surrounding rock and soil. Generally the residence time of groundwater is 
thousands of years, so 

33 c 

See, for example, Menard, Geology, p. 494. 

Constituent or 
physical property 

Chloride (CI) 

Dissolved solids 

Source or cause 


_.. , , - , ... In large amounts in combination with 

Dissolved trom rocks and soils; .. 


present in sewage and found 

large amounts in ancient 

seawater, and industrial 

Chiefly mineral constituents 
dissolved from rocks and 

gives salty taste. In large quantities 
increases the corrosiveness of water. 
Federal drinking water standards 
recommend that the chloride content 
exceed 250 ppm. 

Federal drinking water standards 

Constituent or 
physical property 

Source or cause 

but includes organic matter. 

Hardness as CaCO 3 
(calcium carbonate) 

In most water, nearly all the 
hardness is due to calcium 


Acidity or alkalinity 
(hydrogen ion 
concentration, pH) 

Acids, acid-generating salts, 

free carbon dioxide lower pH. 
Carbonates, bicarbonates, 
hydroxides and phosphates, 
silicates, and borates raise pH. 

Dissolved in water from air and 
Dissolved oxygen (O 2 from oxygen given off in 
) photosynthesis by aquatic 




that the dissolved solids not exceed 500 
ppm. Waters containing more than 1000 
ppm dissolved solids are unsuitable for 
many purposes. 

Consumes soap before a lather will form. 
Deposits soap curd on bathtubs. Hard 

forms scale in boilers, water heaters, 

pipes. Hardness equivalent to the 
bicarbonate and carbonate is called 
carbonate hardness. Any hardness in 

of that is called noncarbonate hardness. 

A pH of 7.0 indicates neutrality in a 

Values greater than 7.0 denote 

alkalinity; values less than 7.0 indicate 

increasing acidity. Corrosiveness of 

generally increases with decreasing pH. 

Dissolved oxygen increases the 

palatability of 
water. Under average stream conditions, 

ppm is usually necessary to maintain a 
varied fish fauna in good condition. For 
industrial uses, zero dissolved oxygen is 
desirable to inhibit corrosion. 

Hunt, Physiography of the United States. 

recharge of seriously depleted aquifers is very slow. 


When the rate of withdrawal of groundwater from an aquifer exceeds the rate of recharge over a 
period of time, the water table falls. The cost of drilling wells and of pumping out the water 
increases rapidly with the depth of the well, so if a water table falls too far below the surface, 
extracting the water becomes altogether uneconomical. In practical terms, the water in such cases 
is a nonrenewable resource that has been mined out. This has happened in parts of Arizona and is 
underway on the high plains of Texas, where extensive use of groundwater has dropped the 

water table as much as 30 meters. Smaller but still serious declines in water tables have 

occurred in other parts of the United States and the world. Such declines not only represent the 
long-term loss of the accessible groundwater resource, but they also can lead to the reduction of 

surface streamfiow, the drying-up of ecologically important ponds and bogs, the intrusion of salt 
water into freshwater aquifers in coastal regions, and subsidence. 


Water quality. An integral part of the issue of availability of water is its quality. Quite 
independent of the pollutants that have been added to water by civilization (which are discussed 
in Chapter 10), the quality of water varies widely because of natural factors. Aspects 

34 Menard, Geology, p. 483; Strahler and Strahler, Environmental, pp. 305-316. 

35 Van Hylckama, Water, p. 153; Menard, Geology, p. 491. 

36 Strahler and Strahler, Environmental, pp. 312-313; J. F. Poland, Land subsidence in the 

western U.S., in Focus on environmental geology, R. W. Tank, ed., Oxford University Press, 

London 1973. 

of quality that determine the usefulness, or even the usability, of water in various industrial, 
domestic, and agricultural applications include color, odor, taste, temperature, oxygen content, 
dissolved salts, and burden of suspended organic and inorganic material. Many of these aspects 
are interrelated, of course. 

The most widespread cause of natural water-quality problems is dissolved salts. Although 
rainwater itself is almost pure, its passage over and through soil and rock formations causes the 
water to accumulate a burden of dissolved mineral material whose chemical composition 
depends in detail on the specific types of soil and rock contacted. Groundwater generally carries 
a higher burden of dissolved material than surface water. Among the most common materials 
dissolved in natural waters are silica, limestone, magnesia, gypsum, and iron. They can cause 
hardness, discoloration, strong tastes, and the formation of troublesome solids in pipes and 
industrial equipment. The properties and consequences of dissolved materials are considered in 
more detail in Box 6-3. 

Federal standards for drinking water specify that the concentration of all dissolved solids in 


combination should be less than 500 parts per million by weight. Water containing as much as 

2500 ppm can be tolerated by livestock. Some industrial uses require water with a considerably 
smaller proportion of dissolved solids than the drinking water standard. Water for irrigation is 
considered excellent to good if dissolved solids are in the range from to 700 ppm, harmful to 
some plants under some conditions in the range from 700 to 2100 ppm, and unsuitable under 


most conditions at more than 2100 ppm. By comparison, seawater contains about 35,000 ppm 

dissolved solids and the Great Salt Lake, nearly ten times that much. 

A high content of dissolved solids in natural waters is a much greater problem in the Southwest 
and the northern plains than elsewhere in the United States ( Figure 6-7 ) . This condition results 
from the high leachability of the native rocks and soils in those areas, a slow rate of runoff 
(maximizing the contact of a given quantity of water with soil and rock), and a high evaporation 
rate (concentrating the salts in the diminished water volume left behind). The naturally high 
salinity in the Southwest has been increased further by certain activities of civilization: the 
construction of reservoirs has increased the evaporation rate, and the use and reuse of water for 

irrigation has increased both the evaporation and leaching rates. The climbing salt content in the 
lower Colorado River has caused political friction between states and between the United States 
and Mexico. ^_The inability of present water-users in this drainage to tolerate further increases in 
dissolved salts may limit the exploitation of western coal and oil-shale, because the water- 
intensive processes involved in extracting, processing, and transporting these resources would 
increase both evaporation and leaching of salts (see Chapter 8). 

Patterns of Use and Supplies 

Human and animal needs for drinking water are dwarfed by the quantities of water used for 
washing and flushing, and they in turn are dwarfed by the amount of water used in 
manufacturing and agriculture. The amounts of water needed for several functions and tasks are 
listed in Table 6-7. Most of the enormous water requirement for crop production is, of course, 
water the crop plants evaporate in conducting their life processes, not free water contained in the 
harvested plant or water fixed in the plant's carbohydrate. The even larger amount of water used 
for milk and beef production is almost entirely needed to grow the feed for the cows and steers. 
Of special importance in determining the water needs of an expanding world agriculture is that 
crops grown in and or nutrient-deficient regions require more water per unit of edible produce 
than those grown in the more favorable conditions characterizing most agriculture today. — 

To examine patterns of water use in detail, it is necessary to distinguish among three phenomena: 
(1) withdrawal of water from surface or underground flows and reservoirs, wherein part of the 
water may be returned after use to the same flows and reservoirs and can be used again; (2) 
consumptive use of some of the water that has been withdrawn ~ usually by evaporation but 
sometimes by embodying the water in a product or polluting it 


Hunt, Physiography, p. 80. 
Hirshleifer et al., Water supply, p. 23. 
39 Menard, Geology, p. 494; Hirshleifer et al., Water supply, p. 23. 
Borgstrom, Too many, p. 138. 
-264- PAGENUMBER265 

FIGURE 6-7 Burden of dissolved solids in U.S. rivers. Numbers indicate parts per million. 

(Adapted from Hunt, 1967.) 

beyond possibility of reuse; (3) flow requirement, wherein the water is used in place for dilution 
of wastes, generation of hydropower, navigation, maintenance of habitat and wildlife, and 

Withdrawal and consumption. The fraction of a withdrawal that is actually consumed varies 
widely among the different sectors of water use. For electric utilities, which use water mainly for 
carrying away the waste heat inevitably produced at power plants (see Chapter 8), the 
consumption was only 0.6 percent of withdrawals in the United States in 1970. 41 The figure is so 
low because most of the cooling was by the oncethrough technique, in which cool water from a 
river, lake or ocean is warmed 8° to 10° C and then returned to its source. Although the water is 
not consumed in the 

TABLE 6-7 

Some Water Requirements 


Drinking water (adult, daily) 

Toilet (1 flush) 

Clothes washer (1 load) 

Refine a ton of petroleum 

Produce a ton of finished steel 

Grow a ton of wheat 

Amount of water used(m ) 







Use Amount of water usedfm ) 

Grow a ton of rice 1,500-2,000 

Produce a ton of milk 10,000 

Produce a ton of beef 20,000-50,000 

Note: 1 m = 1000 liters = 264 gal. Tons are metric tons 
(= 1000 kg). 

Source: Georg Borgstrom, Too many, p. 139; J. J. DeHaven Hirschleifer, 
and J. Milliman, Water supply, p. 27. 


This and other 1970 United States figures in the next few paragraphs we from C. Murray and 
E. Reeves, Estimated use of water in the United States in 1970. 


C;< I- I Hi.' 




FIGURE 6-8 Freshwater withdrawals in the United States, 1970. The upper numbers represent 

surface water; the lower numbers, groundwater. All numbers represent billions of gallons per day 

(the annual amounts divided by 365). (Data from Murray and Reeves, U.S. Geological Survey, 

circular 676 quoted in Strahler and Strahler, 1973.) 

process, its cooling capacity is effectively gone until it has managed, perhaps some distance 
away, to give up the added heat to the atmosphere. The consumed fraction of electric utility 
water withdrawals will increase sharply in the future, however, because regulations restricting 
thermal discharges to rivers, lakes, and the ocean are forcing power plants to rely more heavily 
on evaporative cooling systems. 

Industry consumes about 1 1 percent of the water it withdraws, a fraction that is expected to 
decrease in the future as economic and regulatory incentives encourage repeated reuse of water 
(recirculation), once withdrawn. ^Domestic and commercial consumption of water from 
municipal systems is about 22 percent of withdrawals. Irrigation, by far the largest actual 
consumer of water in the United States, consumes about 75 percent of the water it withdraws. As 
noted above, moreover, much of the water returned after irrigation carries a bigger burden of 
dissolved salts than the water withdrawn. 

Withdrawals of fresh water for all purposes in 1970 in the United States are shown in Figure 6-8 , 
broken down into drainage basins and into withdrawals from surface and groundwater. Notice 
that most withdrawals in the Northwest and Northeast are from surface water, whereas 
withdrawals from groundwater are roughly equal to those from surface water in the Southwest. 

Table 6-8 presents, for the same geographical divisions of the country, data for total withdrawals 
and consumptive use compared with mean annual runoff and annual runoff exceeded 9 years out 
of 10. All these data are annual flows divided by 365, to give mean daily flows in millions of 
cubic meters. Naturally, the runoff figures for each region represent water that originates there 
and do not include inflow from upstream regions (for example, flow into the lower Colorado 
basin from the upper 


National Commission on Materials Policy, Material needs and the environment today and 
tomorrow, pp. 8-5-8-6. 

TABLE 6-8 Freshwater Runoff, Withdrawals and Consu 
Hydrologic Regions 

imption in 

North American 

Quantities (million 

m /day) 




exceeded 9 

years out of 10 



Columbia-North Pacific 





California- South Pacific 





Great Basin 





Upper Colorado 





Lower Colorado 





Rio Grande 















Texas (Gulf) 










Upper Mississippi 





Lower Mississippi 





Quantities (million m /day) 




^xceeded 9 




years out of 10 



Great Lakes 















New England 





Middle Atlantic 





South Atlantic (Gulf) 







Totals 4541 2831 

Note: All flows are mean daily figures (annual flows + 365). 

Source: C. Murray and E. Reeves, Estimated use of water in the United States in 1970. 
TABLE 6-9 Water Users in the United States 

Quantities (million m /day) 









Public municipal 





Rural domestic 





Electric utilities 





Self-supplied industry 










Note: All figures are mean daily flows (annual 


Source: C. Murray and E. 

Reeves, Estimated use of water in the 

United States in 


Colorado River). Of eighteen regions, there is 1 (the lower Colorado) in which consumptive use 
exceeds mean annual runoff, 3 in which consumptive use exceeds the flow available 9 years out 
of 10, and 6 in which total withdrawals exceed flow available in 9 years out of 10. These are the 
main potential trouble spots for water supply in the United States. 

Not represented in Figure 6-8 or in Table 6-8 are withdrawals of salt water amounting to about 
200 million cubic meters per day nationwide, mostly for cooling electric power plants. 

Table 6-9 shows the breakdown of freshwater withdrawals and consumptive use in the United 
States in 1970 according to the category of use: municipal(in- 


- 4 ; 





: : 

j ;-■[■ 





■ TlfBS 























FIGURE 6-9 Irrigated land area of the globe (million hectares), 1963-1965. (Data from 

Borgstrom, 1969.) 

eluding domestic, commercial, and some industrial uses), rural domestic, electric utilities, other 
self-supplied industrial, and irrigation. Irrigation accounts for about 40 percent of withdrawals 
but 87 percent of the consumptive use. 

Projections made for the year 2000 by the United States Geological Survey show a doubling of 
withdrawals over the 1970 figures but an increase of about 50 percent in consumptive use of 
water, implying a greater degree of reuse. 43 Almost half the projected increase in withdrawals is 
expected to be used for cooling electric power plants; a slower rate of growth in electricity 
generation, which now seems likely (see Chapter 8), would significantly reduce those projected 
withdrawal requirements. 

Even the higher forecasts of water use might be met by the national water supply, but they would 
certainly aggravate existing regional difficulties in the drier parts of the country. That shortages 
of water already exist in the arid regions bodes ill for schemes to develop their massive fossil- 
fuel resources and to expand irrigated land for food production. Nevertheless, on a national basis 
problems of water quality will probably remain more important than problems of absolute 

Irrigation. As the biggest consumer of water and a cornerstone of agricultural production in 
many parts of the world, irrigation deserves special attention. The practice of irrigation appears 
to be as old as agriculture itself. ^_The most rapid expansion of the area under irrigation has 
probably been in the past two centuries, however. In the nineteenth century, the amount of land 
under irrigation worldwide increased from about 8 million to 45 million hectares. By the mid- 

1960s the irrigated area was about 180 million hectares, distributed as indicated in Figure 6-9 . 
This amounted to about 5 percent of all cultivated land. 

In the United States, only about 10 percent of the harvested acreage is irrigated, but that land 
produces more than 25 percent of the cash value of crops grown. — 

43 National Commission on Materials Policy, Material needs, p. 8-5. 

44 Borgstrom, Too many, pp. 184-185, is the source of most of the information in this paragraph. 
See also Eric Eckholm, Losing ground, chapter 7. 
5 Edward Groth, III, It 

Edward Groth, III, Increasing the harvest. 

The importance of irrigated land worldwide is probably similarly disproportionate to its fraction 
of the total cultivated area. 

The amount of irrigated land worldwide in the midl970s was about 200 million hectares, out of 
perhaps 340 million hectares that are considered potentially irrigable. 46 The extension of 
irrigation to the remainder is expected to cost at least $1200 per hectare on the average, the price 
naturally rising as the more difficult areas are developed. ^_The quality of existing irrigation 
systems varies widely, moreover, and some land already counted as irrigated needs more water 
than the existing systems deliver if it is to be fully productive. 

Waterlogging and salt accumulation on irrigated land in many parts of the world make it 
questionable to some observers whether optimistic projections of the continued expansion of the 

land area under irrigation will be realized. Iraq, Pakistan, and India have each suffered severe 

damage by waterlogging and salination to millions of hectares of irrigated land, and a noted 
Soviet soil scientist goes so far as to assert that 60 to 80 percent of all irrigated lands worldwide 
are becoming gradually more saline, hence less fertile. 49 Sophisticated drainage technology 
could alleviate this problem in many circumstances, but it increases the expense and requires 
highly trained engineers to implement. ^Paradoxically, failure to get rid of irrigation water 
properly once it has been provided may in the long run prove to be a bigger threat to world 
agriculture than not providing enough. 

Flow requirements. The figures discussed so far have included withdrawals and consumption. 
They have not included the various uses for flow in stream channels that were identified earlier. 
Of these so-called flow requirements, the most demanding under most circumstances is for the 
dilution of wastes. The criterion generally used to determine the flow requirement for waste 
dilution is that the amount of dissolved oxygen in the water should not drop below 4 milligrams 
per liter (4 ppm, by weight). ^_This is the amount needed under most conditions to maintain a 
varied aquatic fauna. 

Obviously, the amount of flow needed to dilute wastes to concentrations such that their bacterial 
or chemical decomposition does not deplete dissolved oxygen below this level depends directly 
on how much waste is discharged. The most commonly used quantitative measure of waste 
material in water is biochemical oxygen demand (BOD), which refers to the weight of oxygen 

needed to oxidize the wastes. A common unit of BOD is the population equivalent (the 0.1 13 kg, 
or 0.25 lb, of oxygen needed to oxidize the daily wastes of a "standard" urban person). — 
Percentage of treatment means the fraction of the BOD that is removed by waste-processing 
before the effluent is actually discharged to waterways. 

If treatment of municipal wastes in the United States in 1960 averaged 70 percent and that of 
industrial wastes, 50 percent, the dilution requirement then would have been 2300 million cubic 

meters of water per day. This compares to a daily flow (the annual total divided by 365) 

exceeded 9 years out of 10 of 2800 million cubic meters. Thus, counting both flow requirements 
and consumption (the latter about 300 million cubic meters per day), most of the reliable flow in 
the forty-eight contiguous states was already being used by society in 1960. 

Most forecasts of future flow requirements for dilution of wastes assume extremely effective 
treatment~95 percent or more for both municipal and industrial wastes. ^_This means all wastes 
would receive tertiary treatment, which would require heavy investments in the construction of 
new treatment plants (see Chapter 10). Such forecasts must also estimate levels of population 
growth and economic growth, as well as the mix of industrial activities. Obviously, there are 
many possibilities, but all credible scenarios indicate substantial water deficits over most of the 
southwestern and south-central United States by the year 2020, even with 97-percent waste 


46 University of California Food Task, A hungry world, p. 71. 

47 ibkL 

See, for example, Eckholm, Losing ground Chapter 7; Borgstrom, Too many, Chapter 8; see 
also Chapter 1 1 in this volume. 
V. Kovda, quoted in Eckholm, Losing ground, p. 124. 
50 For an interesting discussion of what seems possible in theory, see R. Revelle and V. 
Lakshminarayana, The Ganges water machine. 

Hunt, Physiography, p. 83; H. Landsberg, L. Fischman, and J. Fisher, Resources in America's 

An excellent discussion of water quality and waste treatment is N. Wollman and G. Bonem, 
The outlook for water, chapter 8. See also Metcalf and Eddy, Inc. Wastewater engineering, 
McGraw-Hill, New York, 1972, and Chapter 10 in this volume. 

Wollman and Bonem, The outlook. Accurate figures for what the avenge percentages of waste 
treatment actually were in 1960 do not seem to be available, but according to Wollman and 
Bonem they probably were no higher than those given here. 
See, for example, Ridker, Future water needs. 

everywhere. High-growth scenarios extend the deficit region throughout the north-central region, 
as well. 55 Failure to achieve higher levels of treatment, if such failure is coupled with substantial 

48 contiguous states before the year 2000. — 

economic growth, could cause national dilution requirements to exceed mean total runoff for the 
48 contiguous states bef 

Increasing the Supply 

Several approaches to alleviating water shortages are possible: (1) One can redistribute the flow 
more evenly in time by building storage reservoirs or, in some circumstances, by revegetating 
denuded regions so that water is released more gradually. (Both of these strategies buy increased 
regularity of flow at the cost of a decreased total quantity of flow, owing to increased losses to 
evaporation.) (2) One can redistribute the water geographically by transferring it in pipelines or 
canals from one drainage basin to another. (3) One can try to move the people to the water— that 
is, to employ land-use planning tools to concentrate the population where the water is. (This 
approach is not considered further in this chapter; some discussion of land-use planning appears 
in Chapter 14.) (4) One can tap groundwater more extensively by means of wells. (5) One can 
render seawater or salty groundwater suitable for municipal, industrial, and agricultural use by 
desalting. (6) One can try to increase the efficiency with which water is used, for example, by 
minimizing the number of gallons needed to grow a given amount of cotton or to manufacture a 
given amount of steel. 

Water projects. The extent to which water projects had increased the regularity of the flow of 
water in the United States had reached about 40 percent of the theoretically attainable figure by 


around 1970. The construction of dams for this purpose (and others) is expensive and becomes 

increasingly so per unit of control gained, as the best sites are used first. Costbenefit analysis to 
determine whether it is economical to build a large multipurpose dam (for flow regulation, flood 
control, hydropower generation, irrigation, recreation) is complicated. It is easily botched or 
misused by special interests, even independent of environmental issues and the difficulty of 


incorporating these into an economic framework. Not surprisingly, then, costbenefit analyses 

of water projects have generated many controversies. !!_(The environmental aspects of dams are 
considered briefly in connection with a discussion of hydropower in Chapter 8 and in more detail 
in Chapter 1 1 .) 

Diversion of water from regions of surplus to regions of deficit raises all the economic and 
environmental issues of flow-regulation projects, and more. Such interbasin transfers are 
common in the western United States, with 20 percent of the population in the seventeen western 
states already being supplied by water moved 160 kilometers or more in 1968. ^_As the 
distances water is moved grow larger, the associated political difficulties often intensify. People 
in densely populated, water-short regions like southern California use the political power of their 
numbers to exert a claim on water from elsewhere (northern California, in this case), where the 
inhabitants see the environmental costs but receive few benefits of exporting water from their 
region. Few projects that transferred water across state lines existed in the United States as of the 
mid-1970s, but projects had been proposed (and were being taken seriously by some people) for 
moving water to arid parts of the United States from as far away as northern Canada. 61 The 
Soviet Union has long had plans to divert to the south the flow of several major rivers now 
emptying into the Arctic Ocean. ^_Those and other large-scale diversion schemes 

55 Ibid. 

56 This forecast is based on an average of 80-percent treatment of both industrial and municipal 
wastes and the high-growth scenario of Woll man and Bonem, The outlook, p. 69. The low- 
growth scenario with the same treatment levels produces the same result before 2020. 


Ridker, "Future water needs", pp. 215-218. The theoretical maximum is expressed as a 

quantity of flow maintainable 98 percent of the time if all possible storage facilities are built. 
58 See, for example, Hirshleifer et al, Water supply, p. 23, for a good discussion. 
59 T. Parry and R. Norgaard, Wasting a river, and Julian McCaull, Dams of pork. 
60 Menard, Geology, p. 495. 
61 This scheme, the North American Water and Power Alliance, is described unenthusiastically 

in Menard, Geology, p. 496, and does not seem to have much support at this writing. See also 

Paul R. Ehrlich and John P. Holdren, Population and panaceas : A technological perspective; 

and W. Sewell, NA WAP A: a continental water system. Canadians view the scheme with 

considerable misgivings; see R. Bocking, Canada's water: for sale? 

P.Micklin, Soviet plans to reverse the flow of rivers; M. I. Goldman, Environmental pollution 

in the Soviet Union. 

would be staggeringly expensive. (The estimate for the North American Water and Power 
Alliance—the scheme for transferring water from Canada, mentioned above-was $100 billion in 
the 1960s and would surely be much higher if reexamined in the light of more recent experience 
with construction costs.) The ecological effects might well extend to impact on continental and 
global climates (see Chapter 11). 

Desalting. Some 700 desalting plants were in operation around the world in 1975, most of very 
modest size and almost none used to support irrigation of staple crops. The largest ones produced 
on the order of 35,000 cubic meters of fresh water per day, at a cost of around 15 cents per cubic 
meter. 63 In smaller plants, the cost was in the range of 25 to 50 cents per cubic meter. By 
contrast, farmers typically pay I to 2 cents per cubic meter for irrigation water. So, only for 
growing high-value crops such as tomatoes, avocados, and orchids can water from desalting be 
used economically for agriculture. It appears that desalting is too expensive by about a factor of 
10 to permit its use for irrigating wheat, rice, or corn. M 

Some desalting enthusiasts argue that a combination of lower future costs for desalting and 
reduced water requirements for staple crops (made possible by more refined irrigation schemes 
and by breeding more drought-resistant, water-efficient crop varieties) will soon change this 
verdict, opening an era in which the deserts will be made to bloom. ^_Most such analyses appear 
to rest on the hope of cheap nuclear power as a source of energy for the separation of salt from 
seawater. (The theoretical minimum amount of energy needed to remove the salt from a liter of 
seawater, which can be established using basic principles of thermodynamics, is a not too 
formidable 2.8 kilojoules. The most efficient plants actually constructed to date require about 
170 kilojoules per liter for practical reasons not likely to be circumvented easily or soon.) ^_The 
price of nuclear energy has been rising, not falling, however, and other energy sources are 
becoming costlier, too (see Chapter 8). At one dollar per million kilojoules, a typical price for 
"low-cost" commercial fuel energy in 1975, the energy cost alone for desalting a cubic meter of 
seawater in an efficient plant was 17 cents. 

Falling construction costs for large desalting plants were also rather widely predicted in the 
1960s, but those have not materialized, either. ^Moreover, if the water is to be used anywhere 
but on the seacoast (or directly adjacent to inland supplies of salty water), the costs of moving it 
and lifting it in elevation must be taken into account. Lifting costs alone are on the order of 1 

cent per cubic meter per hundred meters of lift. ^JUnder these conditions, one could scarcely 
afford to use seawater for irrigating staple crops on land above a few hundred meters elevation, 
even if the sea were fresh water to start with. 

Water conservation and other strategies. The use of wells to develop groundwater supplies 
can in some instances be an economical alternative to dams and surface reservoirs for the 

purpose of flow regulation and storage, but any situation in which withdrawals exceed natural 

recharge is obviously not a satisfactory long-term strategy. 

As with so many other resource problems, close examination of the water situation suggests that 
the most economical and environmentally benign way to increase supplies beyond a certain point 
is really not to increase supplies at all, but rather to reduce consumption by more efficient use. A 
substantial fraction of municipal water use in the United States, for example, turns out to be the 


result of leaks, including dripping faucets, running toilets, and the like. Furthermore, industrial 

water use could be significantly reduced by increasing recirculation at the expense of 

7 1 

withdrawals. There is genuine potential, as well, for reducing significantly the amount of water 


needed for productive irrigation. " At least one approach to this—the use of high-frequency 
irrigation (delivering small amounts of water at frequent intervals) 

63 Van Hylckama, Water resources, p. 161. The cost is based on energy prices before 1973 and 

could hardly be correct in 1977. 

M. Clawson, H. Landsberg, and L. Alexander, Desalted seawater for agriculture: Is it 


See, for example, Gale Young, Dry lands and salted water. 
66 J. Harte and R. Socolow, Patient Earth, p. 272. A more thorough introduction to standard 

desalting technology appears in Hirshleifer et al., Water supply, pp. 203-217. 

See, for example, Ehrlich and Holdren, Population and panaceas . 

Van Hylckama, Water resources, p. 161. 
69 See, for example, Revelle and Lakshminarayana, The Ganges. 


Water Resources Council, The nation's water resources. 

7 1 

National Commission on Materials Policy, Material needs, chapter 8. 


See, for example, Young, Dry lands. 

through pipes—is said by its supporters to be capable of increasing the efficiency of fertilizer 

uptake and increasing the variety of soils on which irrigation is possible. Breeding crops more 

tolerant of salt is another way to reduce the use of fresh water in agriculture, but it is a time- 
consuming and as yet unpromising approach. 

Naturally, the lengths to which society will go to use its water more efficiently depend in part on 
the price of water, and many economists believe that many present water problems in the United 
States— caused in large part by excessive consumption— could have been avoided if the price of 
water had not been kept artificially low by subsidies. 74 It is likely to prove necessary eventually, 
either by means of much higher prices or by other social restraints, to prevent people from 

moving into arid regions with the expectation that society will support their venture by making 
the necessary water available at negligible direct cost to the consumer. 


With the disappearance of the forest, all is changed. At one season the earth parts with its 
warmth by radiation to an open sky— receives, at another, an immoderate heat from the 
unobstructed rays of the sun. Hence the climate becomes excessive and the soil is alternately 
parched by the fervors of summer and seared by the rigors of winter. Bleak winds sweep 
unresisted over its surface .... The face of the earth is no longer a sponge, but a dust heap, and 
the floods which the waters of the sky pour over it hurry swiftly along its slopes, carrying in 
suspension vast quantities of earthy particles which increase the abrading power and 
mechanical force of the current, and, augmented by the sand and gravel of falling banks, fill the 
beds of the streams, divert them into new channels, and obstruct their outlets. 

— George Perkins Marsh, Man and nature, 1864 

Intimately related to freshwater supplies are another renewable resource—forests. Forests are not, 
of course, the same from place to place—as was seen in Chapter 4, the term forest has a multitude 

of meanings. At the same time, diverse as they are, forests the world over tend to play similar 

roles in relation to human activities: maintenance of ecological diversity, preservation of 
watersheds and the prevention of erosion (and subsequent siltation of dams), moderation of 
climate, supplying wood for fuel and structures and paper, and providing hunting grounds and 
areas of aesthetic value for recreational purposes, to name a few. 75a The multiple uses of forest 
land are often to one degree or another incompatible, but, for any or all of them to continue, 
preservation or regeneration of forests is necessary. Therefore, much of the discussion on the 
uses of forests tends to focus on what happens when they are destroyed— that is, on deforestation. 

That deforestation results in heavy soil erosion, floods, and changes in local climate has been 
known— but not always heeded— for centuries. The annual floods that have plagued northern 
China since ancient times are a result of deforestation during the early dynasties. ^_The once- 
fertile hills of central Italy have been arid and subject to regular, occasionally devastating floods 
since the Middle Ages, when the trees were removed. It is interesting that medieval writers and 
many others since accurately predicted the results of deforestation without replanting or 
provision for reseeding and protection of seedlings and saplings. The ancient Greeks and 
Romans apparently were relatively conscientious in maintaining the forests and understood their 
value in protecting watersheds. But this understanding seems to have been partially lost during 
the Middle Ages, when the demands of growing populations for fuel, construction materials, and 
grazing land brought destruction of the forests in much of southern Europe. This destruction led 
the great naturalist Alexander von Humboldt to observe, in the middle of the nineteenth century, 


that the youthfulness of a civilization is proved by the existence of its woods. — 


S. L. Rawlins and P. A. C. Raats, Prospects for high-frequency irrigation. 


See, for example, Hirschleifer et al., Water supply. 

A fine introduction into the diversity and complexity of forests appears in S. H. Spurr and B. 

V. Barnes, Forest ecology. 
75a For a splendid summary of the underappreciated values of forests to human society, see F. H. 
Bormann, An inseparable linkage. 
Borgstrom, Too many, p. 2. 


An excellent recent perspective on historical and contemporary forest problems around the 
world is given in Eckholm, Losing ground, chapter 2. For the consequences of deforestation 
in Italy, see Richard M. Klein, The Florence floods . 
-272- PAGENUMBER273 

TABLE 6-10 World Forests 

Areas (million km ) Timber Stocks (billion m ) 



















































































World 135.6 40.3 28.0 20.1 224.0 102.9 121.1 341 

"Land where crowns of trees cover more than 20% of area. 
Entirely within closed forests. 

c Total of coniferous and broad-leaf, closed forest and other. 
Source: Reidar Persson, World forest resources. 
World Forest Reserves 

Today, similar demands are threatening forests around the world. Many irreplaceable tracts have 
disappeared entirely. Most of Europe, northern Asia, the eastern one-third of the United States, 

and vast areas of the American Northwest were once covered with forests. Only fractions of the 
forests of the eastern United States and of western Europe remain today, largely preserved 
through conscious conservation and reforestation policies, which were pioneered in Europe. 
Table 6-10 shows forested areas and estimated timber stocks for the world as of 1973. 

The Soviet Union has the greatest remaining reserves of temperate and subarctic forest, including 
nearly half of the world's coniferous forest. About two-thirds is virgin, largely because it is 
relatively inaccessible. However, the best-quality trees for lumbering are in the European USSR, 
and as a result, those have been heavily exploited. Large reserves of coniferous forest also 
remain in North America, from Nova Scotia to Alaska. Forest management practices have, in 
theory, been established in most DCs, including the USSR and the United States, to preserve 
their forest for the future—but the success of such practices is in dispute. Interestingly, problems 
and conflicts over timber harvesting in the communist Soviet Union are quite similar to those 
that are so vexing in the capitalist United States. — 

Australia and New Zealand in recent decades have also lost large tracts of forest, and much of 
what remains is being heavily exploited by intensive forestry. In addition, where replanting is 
done, native forest ecosystems are replaced with tree-farms of fast-growing, imported species 
harvested by broad-scale clearcutting. — 

Vast forest reserves also still exist in the tropics, especially in the Amazon valley, southeast Asia, 
and central Africa. Inaccessibility and economic factors have until recently protected those areas 
from destruction. Latin America, for example, harvested less than 4 percent of the wood 
produced worldwide in 1967-1969, despite having almost a third of the world's reserves. — 
Nevertheless, some of Latin America's more accessible forests have now vanished or have been 
selectively depleted of the most valuable species. The rate of clearing tropical rain forests has 
accelerated in the past decade or so to the point where many fear they will all but disappear by 
the end of the century. 

It is important to recognize that the greatest cause of outright deforestation in the world today is 


Philip R. Pryde, Conservation in the Soviet Union; M. I. Goldman, Environmental pollution in 
the Soviet Union. 


R. and V. Routley, The fight for the forests. 
E. P. Cliff, Timber -c 

E. P. Cliff, Timber-an old, renewable material. 

certainly not large-scale commercial lumbering, but rather a combination of land-clearing for 
agricultural production and the gathering of wood by desperate peasants in need of fuel. While 
debates rage in the DCs about multiple-use and optimum harvesting practices, the very existence 
of forests is in jeopardy over large expanses of the world, where hunger and the need for warmth 
take precedence over more subtle concerns. — 

Forests in the United States 

In the United States, lumber interests are accelerating their harvests, often at considerable 
expense to the forest environment, in their efforts to meet rising demand for construction wood 
and for paper. Table 6-11 shows both annual growth and annual removals of timber in the United 
States from 1952 to 1970, with projections to the year 2000. While growth has exceeded 
removals overall and may continue to do so, softwood sawtimber (coniferous trees more than 12 
inches in diameter) has suffered serious deficits, which are expected to continue. Thus, the stock 
of those trees, which supply most of the wood for construction and pulping, is dwindling. ^_How 
much of the demand for wood in the United States (in the economic sense, the desire and ability 
to purchase it) is related to real needs is in dispute. For instance, if wood is cheap enough, the use 
of it as a structural material will increase and the demand for nonforest products (metals, 
concrete) that can meet the same needs will decrease. Much of the pressure generated by the 
industry for more logging in the national forests appears to have been created by a fear of loss of 
markets to other industries rather than by any genuine national need. _ On the other hand, the 
United States has been a net importer of timber since 1941, and the fractional dependence on 
imports is growing. — 

The U. S. lumber industry. Few subjects have been the source of more enduring environmental 
controversy in the United States than the needs and practices of the 

TABLE 6-11 Annual Growth and Removal of Wood in the United States, 1952-2000 






































































Note: Figures for 1980, 1990, and 2000 are projections. 

This category is included in the other. Growing stock is trees 4 
inches or more in diameter. Sawtimber is trees 12 inches or more in 
diameter. A board foot is 12 in. x 12 in. x 1 in. 

Source: National Academy of Sciences, Man, materials, and 
environment,^. 135. 

lumber industry. The controversy goes back to the middle of the nineteenth century, when the 
wholesale plundering of American forests was in full swing, due both to unscrupulous industry 
practices and to irresponsible farm clearing. A few voices protested—John Muir, Henry David 
Thoreau, Carl Schurz ( President Hayes' Secretary of the Interior)~but the demolition of the 

forests continued. In 1907 establishment of the Forest Service under Gifford Pinchot, a man 

who believed that exploitation and preservation of forests could be compatible, marked a turning 
point. The trend began toward sustained-yield forestry—harvesting trees like other crops— and 
toward consideration of nonlumbering uses of forest such as for wilderness, watershed 
preservation, and recreation. 

That hopeful trend, however, has not led to any sort of equilibrium based on a comprehensive 
American forest policy. The timber industry, for instance, constantly pressures the Forest Service 
to open up more national 

Eckholm, Losing ground chapter 6. 

Q 1 

National Academy of Sciences, Man, materials, and environment, chapter 5. 
82 N. Wood, Clearcut, p. 26. 
Cliff, Timber, see also National Commission on Materials Policy, Material needs, pp. 2-22. 


For a brief summary of the history of U.S. forests see Wood, Clearcut, p. 34 ff. 
-274- PAGENUMBER275 

forest for harvesting— pressure that succeeds all too often. ~ At the same time pressures for 
recreational use are escalating, and the general public concern for conservation is increasing.The 
degree of compatibility of different uses varies a great deal, ^_and decision-making in this area is 
complex, to say the least. In general, however, decisions by the Forest Service (and other 
government groups) have tended to favor the lumber interests, even though the Forest Service 
has not become a pawn of those interests to the extent that some other federal agencies (such as 
the Interstate Commerce Commission) have become subservient to the industries they are 
supposed to regulate. The political muscle of the lumber companies and the emergence of a 
generation of Forest Service officials trained to regard forests as crops have nonetheless kept 
timber harvesting at the top of the list of the multiple uses to which those forests are, according 
to law, to be put. National forest policy. That one legitimate and important use of national forest 
land is supplying the United States with wood is not in dispute. But a series of related questions 
are in hot contention. They include: 

How much of our national forest land should be assigned to growing crops of timber, and 

which areas should be put to that use? 

How should demand for export be included in estimates of the need to cut on those lands? 

To what extent should the intensive, high-yield forestry practices employed by industry on 

private lands be permitted on multiple-use public lands? 

What methods of harvesting should be employed? 

These questions are interrelated and can only be answered satisfactorily by a comprehensive 
forestmanagement policy. The way to proceed seems clear-toward a high-intensity, low-harvest- 

acreage policy similar to one suggested by Marion Clawson. The concept is relatively simple. 

Growth of wood for extraction from our forests would be concentrated in the better forest sites 
(those producing the greatest yields and suited to rapid regeneration). Sites less suitable would be 

reserved for alternative uses or held for possible future harvests. The nation's wood would come 
from a much smaller area than if all of today's so-called commercial forests were harvested, and 
some prime timberland now seemingly doomed to be cleared could be set aside for wilderness. 

Such a policy is desperately needed. The demand for forest products seems certain to rise over 
the next several decades; it will do so all the faster if we turn more to forests for fuel sources. In 
1974 it was estimated that demand for wood would rise at a compound rate of perhaps 1 .5 
percent per annum between 1970 and 2000. The rise in recreational demand was projected to be 
much higher—perhaps 4 percent—meaning nearly a quadrupling of use by the end of the century. 


Furthermore, it now seems likely that for economic reasons (for maintaining foreign exchange) 

the United States will want to expand its lumber exports to Japan and elsewhere. 

If there is to be any chance of meeting those pressures on our forests without a national disaster, 
their exploitation must be more tightly controlled— not just at the expense of lumbering interests 
but also through such actions as equitable rationing of access to wilderness, since overuse by 
hikers and campers can destroy the very values conservation intends to preserve. As is so often 
the case, however, even when the direction in which policy should go seems clear, economic and 
political impediments to moving in that direction may be formidable. We return to that in 
Chapter 14. 

Unsound practices. It is often more immediately profitable for a company to harvest timber in a 
way that essentially converts forest into a nonrenewable resource than to harvest in what may be 
the soundest method, taking the economic or environmental long view. In a sense, many forests 
have been treated as "terrestrial whales"— harvested with no consideration for maximum 
sustainable yield but rather with an eye on maximum return on capital (Chapter 7). For example, 
many areas 

See, for example, John Hart, Assault on the Siskiyous. For a more general discussion see the 
section on timber in Council on Environmental Quality, Environmental quality, 1975, pp. 
86 For a good discussion of the relationships among uses we M. Clawson , "Forests: For whom 
and for what?" 

Conflicts, strategies and possibilities for consensus in forest land use and management. 
Demand estimates are based on L. L. Fischman, Future demand for U.S. forest resources. 

have been subjected to the wrong kind of selective cutting. The most desirable species have been 
logged out, leaving only inferior trees to reseed. The result is an inferior forest, at least from the 
point of view of future harvests. This has been the fate of much hardwood forest in the eastern 
United States. — 

This type of selective cutting is often disastrous, but the disaster is not as obvious as where there 
is clearcutting (the wholesale removal of large tracts of mature forest). Done carefully, with 
appropriate care for aesthetics, placement of roads, removal of logs, and disposal of trash, and 
done in forest ecosystems with the appropriate capacity to regenerate (such as the Hubbard 

Brook forest discussed in Chapter 1 1), clear-cutting may be an appropriate way of harvesting 
timber. Unfortunately, much clear-cutting is done in inappropriate areas, that is, where the result 
is an impression of vast desolation in formerly scenic places, where placement of roads and 
dragging of logs play havoc with the soil, where silting of streams destroys valuable populations 
of game fishes, and where climatic and other factors make reforestation extremely slow or 

Such activities have produced results that, through the dissemination of photographs such as 
those in Figure 6-10 , have convinced much of the public that all lumbering is environmentally 
irresponsible. Similarly, the behavior of Georgia Pacific and Areata Redwood companies in 
hurrying to harvest redwood stands that were "threatened" with inclusion in Redwood National 
Park was in the best tradition of the nineteenth-century "rape and pillage school" of forest 
management. 90 Indeed, the assistant to the chairman of Georgia Pacific was in favor of logging 
within national parks! — 

Unfortunately, even under appropriate conditions, clear-cutting and tree farming present 
problems that are analogous in some respects to those found in other forms of intensive 
agriculture. Use of pesticides, herbicides, and fertilizers on forest plots has caused off-site 

environmental problems, for example. Tree farming also produces some of the liabilities of 

other kinds of monoculture. For example, large stands of young trees are more susceptible to 

disease, pests, and fire than are forests containing trees of varied age. 

Loggers defend the practice of clear-cutting large stands on the grounds that certain valuable 
species of trees need direct sunlight and space in which to grow -that is, cleared land. 
Presumably, some less destructive procedure can be used in many cases, such as clearcutting 
small stands, if selective cutting and replanting individual trees is unsatisfactory. A great deal of 
research on appropriate harvesting procedures in various kinds of forest ecosystems is badly 
needed. At the rate forests are being cut in some areas, however, the forests may be gone before 
the research can be done. 

Forests are threatened by more than a demand for lumber, fuel, and recreation. Much privately 
owned forested land (about 80 percent of the forest in the eastern United States) disappears each 
year under highways, housing subdivisions, airports, and other development projects. Strip- 
mining destroys thousands of acres in a process much more destructive of the soil than 
clearcutting. In moist climates erosion and flooding are virtually inevitable after strip-mining and 
are accompanied by severe water pollution. Reforestation is often not attempted (although with 
care it frequently can be achieved). In publicly owned tracts in national forests, under the 
multiple-use policy, trees are cleared for recreational facilities (and many are damaged by the 
crowds of visitors), access roads, powerline cuts, mining activities, and sheep and cattle grazing. 
And in some areas, for example near Los Angeles, trees are being killed by smog. 

Pressures on World Forests 

The situation is far worse in many other countries. Half the trees cut down in the world are used 
for fuel, regardless of their potential value as lumber. ^Clearing for agricultural land, especially 

in the tropics, is another great threat to this resource. Deforestation may be especially disastrous 
ecologically in the tropics. Because 

M. Clawson, Forest policy for the future, p. 183. 

Wood, Clearcut, p. 130. 

Ibid., p. 134 . 

National Academy of Sciences, Man, materials, chapter 5. 

See, for example, Janick et al., Plant science. 


Eckholm, Losing ground, chapter 6. 

FIGURE 6-10 A. Aerial view of the clear-cutting of redwoods near the Redwood National Park 

in Humboldt County, California. Heavy soil erosion and silting of streams usually follow such 

wholesale deforestation, especially on steep slopes like this. (Photo by Don Anthrop , courtesy of 

Friends of the Earth.) B. Another view of clearcutting near the Redwood National Park in 

California, showing debris left by loggers and exposed soil. (Photo by Bruce Colman, courtesy 

of Friends of the Earth.) 

of poor transportation facilities and various economic factors, a great number of the felled trees 
(many of them valuable hardwoods) are not harvested and used as lumber, but are wasted or used 
for fuel. Brazil's forests once covered 80 percent of the country; by 1965 they covered only 58 
percent. ^_A stretch through the Amazon forest is now being cleared for a transcontinental 
highway, which will open up vast areas of the forest for lumbering and agricultural clearing. 
Lumbering in Haiti's tropical forest has long since exterminated mahogany entirely, and that of 
Honduras is nearly gone. Replanting has seldom been practiced in tropical forests in the past, 
although a few countries are now introducing forest-management practices there. If tropical 
forests are not to vanish, as did temperate ones in Europe and the United States, conservation and 
management policies must be established. 

A bright spot in the picture may be provided by the People's Republic of China. More than 30 
percent of China's land area is said to be suitable for reforestation, and a reforestation program is 
in progress. — 

Careful management of forests could assure humanity of the opportunity to benefit from forests 
more or less in perpetuity. Such management would include preservation of trees of mixed 
species and ages as much as possible, to discourage losses from pests, disease, and fire; careful 
selective logging in a long-term rotation scheme or intensive management of clear-cutting in 
appropriate areas; much more effort put into reforestation; and careful protection of the soil. 


The resources we have discussed in this chapter — land, soil, water, forests — have for the most 
part not commanded the attention or the sense of urgency in the United States, either of the 
public at large or among dedicated environmentalists, that they deserve. Perhaps soil erosion and 
deforestation lack the glamour of the petroleum crisis or running out of tungsten, but the central 
role of the soil-water- forest complex in the support of industrial and agrarian societies alike 
means that high priority should be attached to understanding it and maintaining its integrity. If 
the United Nations conference on the environment in Stockholm in 1972 is any guide, the LDCs 
may understand this better than the DCs — for the lists of environmental problems raised by the 
poor countries at that meeting were heavy with threats to soil and forests. Sadly, the LDC 
governments that perceive the gravity of these threats generally are less well equipped to do 
anything about it than are the DC governments, where recognition is dimmer. 

It is true that the wholesale loss of soil and forests is, as a general rule, proceeding faster in 
LDCs than elsewhere, and some might argue on this basis that the relative complacency of DCs 
about their own situations with respect to these resources is justified. It is not. Probably the main 
reason for overconfidence is lack of awareness of the complexity and subtlety of the soil-water- 
forest complex, and hence blissful ignorance of the ways it might be disrupted and the ways 
these disruptions could damage human well-being. It is important to recognize that soil is a 
chemical and biological system as well as a structural foundation for holding up plants; that what 
grows depends not just on adding water and fertilizer to any plot of land, but on widely varying 
characteristics of soils that are themselves the product of biogeochemical processes acting over 
millennia; and that these characteristics can be interfered with, perhaps irreversibly, by human 
activities operating over much shorter periods of time. (Some ways in which this could happen 
are described further in Chapter 1 1.) It is important to recognize, too, how tightly linked are the 
resources of soil and water and forest. Deforestation produces erosion and water pollution and 
makes runoff more erratic, reducing availability of water and causing more erosion. This process 
can become irreversible by altering the environment so drastically that reforestation is 

Society ignores the complexity and fragility of these systems at its peril. 

95 Georg Borgstrom, Hungry planet, 2d ed. ( Macmillan, New York, 1972), p. 347. 
S. Wormian, Agriculture in China. 
-278- PAGENUMBER279 

Recommended for Further Reading 

Clawson, M. 1975. Forests. For whom and for what? Resources for the Future, Washington, 
D.C. Excellent discussion of forest policy, highly recommended. 

Eckholm, Eric. 1976. Losing ground. Norton, New York. An outstanding treatment of land, soil, 
water, and forests and the pressures they are being subjected to because of population growth and 

Harte, J., and R. Socolow. 1971 . Patient Earth Holt, Rinehart and Winston, New York. An 
excellent collection of environmental case histories, covering land-use and water issues, among 

Hunt, C. B., 1974. Natural regions of the United States and Canada. W. H. Freeman and 
Company, San Francisco. Detailed information on soils, water, vegetation, landforms. 

Janick, J., R. W. Schery, F. W. Woods, and V. W. Ruttan. 1974. Plant science. 2d. ed. W. H. 
Freeman and Company, San Francisco. Contains thorough treatments of soils, soil chemistry, 
and their relation to plant productivity. 

Leopold, Luna B. 1974. Water. A primer. W. H. Freeman and Company, San Francisco. 
Illuminating introductory treatment of surface and groundwater. 

Persson, Reidar. 1974. World forest resources. Royal College of Forestry, Stockholm. This is the 
unofficial successor to the World Forest Inventory (WFI) of the Food and Agriculture 
Organization (FAO) of the United Nations. The last WFI was published in 1966 based on data 
gathered up to 1963. Persson, who worked in the FAO's forest division from 1968 to 1972, 
assembled this volume from data compiled through 1973 by himself and others at the FAO. It is 
the most comprehensive and up-to-date study of world forests we know of. 

Additional References 

Albrecht, W. A., 1956. Physical, chemical, and biochemical changes in the soil community. In 
Man's role in changing the face of the earth, Thomas, W. L., ed. University of Chicago Press, 
pp. 648-673. A study of the effects of long-term cultivation on midwestern soils. 

Berwick, Stephen. 1976. "The Gir forest: An endangered ecosystem". American Scientist vol. 64, 
no. 1 (January-February), pp. 28-40. Excellent description of the problems of maintaining a 
semiarid forest ecosystem in the face of expanding human populations. 

Bocking, Richard C. 1972. Canada's water: for sale? James Lewis and Samuel, Publishers, 
Toronto. Well-documented critique of plans for massive water transfers from Canada to the U.S., 
by a Canadian. 

Borgstrom, Georg. 1969. Too many. Macmillan, Toronto. Good treatment of patterns of global 
land use. 

Bormann, F. H. 1976. "An inseparable linkage: Conservation of natural ecosystems and the 
conservation of fossil energy". BioScience, vol. 26, no. 12 (December), pp. 754-759. Excellent 
summary of the values to society of forest ecosystems. 

Brown, Lester R. 1975. "The world food prospect". Science, vol. 190 (December 12), pp. 1053- 
1059. Tells how much land is used in the cultivation of various major crops. 

California Land Use Task Force. 1975. The California land. Kaufmann, Los Altos, Calif., 
chapter 3 . Trends and patterns of land use in a state where competing demands on land for 
agriculture, urbanization, and recreation are intense. 

Carter, V. G., and T. Dale. 1974. Topsoil and civilization University of Oklahoma Press, 
Norman, Okla. Historical and contemporary problems of erosion and desertification. 

Clawson, M. 1974. Conflicts, strategies, and possibilities for consensus in forest land use and 
management. In Forest policy for the future, M. Clawson , ed. Resources for the Future, 
Washington, D.C., pp. 105-191. An interesting discussion of a high-intensity, lowharvest- 
acreage model of U.S. timber strategy. 

1976. "The national forests". Science, vol. 191 (20 February), pp. 762-767 '. Defects in 

present management practices and some suggested remedies. 

ed. 1974. Forest policy for the future. Washington, D.C.: Resources for the Future. Very 

useful collection of articles. 


H. Landsberg and L. Alexander. 1969. "Desalted seawater for agriculture: Is it economic?" 

Science, vol. 165, pp. 1 141-1 148. This article shows desalting for large-scale staple-crop 
agriculture to be far from economic. Developments since 1969 have not altered this conclusion. 

Cliff, E. P. 1975. Timber — an old renewable material. In National materials policies, National 
Academy of Sciences, Washington, D.C. Notes the continuing versatility of wood as a raw 

Council on Environmental Quality (CEQ). 1975. Environmental quality. Government Printing 
Office, Washington, D.C. Extensive data and commentary on land, water, and timber. 

Deevey, Edward. 1960. "The human population". Scientific American, September. Reprinted in 
Ehrlich, Holdren, and Holm, eds., Man and the ecosphere, W. H. Freeman and Company, San 
Francisco, 1971, pp. 49-55. Contains classification of world land area by vegetation types. 

Ehrlich, Paul R., and John P. Holdren. 1969. Population and panaceas: A technological 
perspective. BioScience, vol. 19, pp. 1065-1071. Discusses logistic and economic limitations on 
large-scale water movement and desalting. 

Fischman, L. L. 1974. Future demand for U.S. forest resources. In Forest policy for the future, 
M. Clawson, ed., pp. 21-86. 

Food and Agriculture Organization (FAO). 1960. World forestry inventory. United Nations, 
Rome. Standard reference, now somewhat dated. 

Gerasimov, I. P.; D. L. Armand; and K. M. Yefron, eds. 1971. Natural resources of the Soviet 
Union: Their use and renewal. W. H. Freeman and Company, San Francisco. See the section on 
water, land, and vegetation. 

Gieseking, John E., ed. 1975. Soil components, vol. 1, Organic components and vol. 2, Inorganic 
components. Springer-Verlag, New York. A comprehensive treatment of soil science for 

Giessner, Klaus. 1971. " Der mediterrane W aid im Maghreb" . Geographische Rundschau, 
October 1971, pp. 390-400. Land use and deforestation in Tunisia, Morocco, and Algeria. 

Goldman, M. I. 1972. Environmental pollution in the Soviet Union. The M.l.T. Press, 
Cambridge, Mass. Good coverage of forest practices, water-management projects, and other 

Groth, Edward 111. 1975. "Increasing the harvest". Environment, vol. 17, no. 1 
(January /February), pp. 28-39. Use of land and water in U.S. agriculture. 

Haden-Guest, S., J. K. Wright; and E. M. Techoff eds. 1956. A world geography of forest 
resources. Ronald Press, New York. A comprehensive source book on such topics as principal 
commercial species, but very out-of-date on policy and exploitation. 

Hart, John. 1975. "Assault on the Siskiyous". Cry California, Fall, pp. 3-11. Deplores timber 
industry pressure to open scenic parts of national forests to lumbering. 

Hirshleifer, J.; J. DeHaven; and J. Milliman. 1969. Water supply. University of Chicago Press. 
Good introduction to the economics of water projects for municipal supply and irrigation. 

Hunt, Charles B. 1967. Physiography of the United States. W. H. Freeman and Company, San 
Francisco. Good treatment of soils, water, landforms. 

Hylckama, T. Van, 1975. Water resources. In Environment, Murdoch, W. , ed., Sinauer, 
Sunderland, Mass. pp. 147-165. Up-to-date survey of global water inventories and problems. 

Judson, S. 1968. "Erosion of the land". American Scientist, vol. 56, no. 4, pp. 356-374. 
Informative discussion of the history and present status of erosion around the world, based in 
part on loads of dissolved and suspended material in rivers. 

Klein, Richard M. 1969. "The Florence floods". Natural history, August/September, pp. 46-55. 
An historical account of the consequences of deforestation in Europe. 

Landsberg, H.; L. Fischman; and J. Fisher. 1963. Resources in America's future. Johns Hopkins, 
Baltimore. An encyclopedia of quantitative information about resource supplies and patterns of 
use in the United States, noted here for its treatment of land, water, and forests. Somewhat dated 
but still very useful. 

Marsh, George Perkins. 1864. Man and nature. Reprinted by Harvard University Press, 
Cambridge, Mass. 1965. The first true classic of the American conservation movement. 
Beautifully written. 

McCaull, Julian. 1975. "Dams of pork". Environment, vol. 17, no. 1 (January /February pp. 11- 
27. Critique of construction of dams when not justified by other than short-term political 

McHugh, J. L. 1966. "Management of estuarine fisheries" . In A symposium on estuarine 
fisheries. American Fisheries Society, Washington, D.C, special publication 3. Emphasizes 
importance of estuaries in marine food production, an important dimension of competing uses 
for coastal lands. 

Menard, H. W. 1974. Geology, resources and society. W. H. Freeman and Company, San 
Francisco. Good introductory treatment of soils and the hydrologic cycle. 

Metcalf and Eddy, Inc. 1972. Wastewater engineering. New York: McGraw-Hill. A useful, 
comprehensive text. 

Micklin, P. 1971. Soviet plans to reverse the flow of rivers. In T. Detwyler , ed., Man's Impact on 
Environment. McGraw-Hill, New York, pp. 302-318. 

Murray, C, and E. Reeves. 1972. Estimated use of water in the United States in 1970. United 
States Geological Survey circular 676, Government Printing Office, Washington, D.C. Basic 
information on patterns of water consumption. 

National Academy of Sciences (NAS). 1973. Man, materials, and environment. M.I.T. Press, 
Cambridge, Mass. Good treatment of environmental problems associated with forest products. 

National Commission on Materials Policy. 1973. Material needs and the environment today and 
tomorrow. Government Printing Office, Washington, D.C; June. Contains projections of future 
U.S. water use in homes, commerce, and industry. 

Parry, T., and R. Norgaard. 1975. "Wasting a river". Environment, vol. 17, no. 1 

(January /February), pp. 11-27. Analytic critique of the cost-benefit analysis used by the Army 

Corps of Engineers to justify a dam on a California river. 

Poland, J. F. 1973. Land subsidence in the western United States. In Focus on environmental 
geology, R. W. Tank, ed. Oxford University Press, New York. 

President's Science Advisory Committee. 1967. The world food problem 3 vols. Government 
Printing Office, Washington, D. C Noted here for classification of world land area according to 
suitability for cultivation. 

Pryde, Philip R. 1972. Conservation in the Soviet Union. Cambridge University Press, London. 
Contains chapters on land, water, and forests. 

Rawlins, S. L., and P. A. C. Raats. 1975. "Prospects for high-frequency irrigation". Science, vol. 
188, pp. 604-610 (May 9). An approach to increasing the efficiency of water use in irrigation. 

Revelle, R., and V. Lakshminarayana. 1975. "The Ganges water machine". Science, vol. 188, pp. 
611-616 (May 9). Use of wells and drainage systems to improve water management in the 
Ganges basin. 


Richardson, S. D. 1966. Forestry in communist China. Johns Hopkins, Baltimore. Somewhat 
out-of-date, but still an important source, especially on such slowly changing subjects as soils 
and major tree species. 

Ridker, R. G. 1972. Future water needs and supplies. In Research reports, vol. 3, R. Ridker, ed., 
Commission on Population Growth and the American future. Government Printing Office, 
Washington, D.C. Useful quantitative treatment of U.S. water situation, with projections to the 
year 2000. 

Routley, R., and V. Routley, 1975. The fight for the forests. 3d ed. Research School of Social 
Sciences, Australian National University, Canberra. 

Sanchez, P., and S. Buol. 1975. "Soils of the tropics and the world food crisis". Science, vol. 188, 
pp. 598-603 (May 9). Problems and potential of tropical soils. 

Sewell, W. 1967. "NAWAPA: A continental water system". Bulletin of the Atomic Scientists, 
vol. 23, no. 7, pp. 8-13. Description of the dormant North American Water and Power Alliance. 

Spurr, S. H., and B. V. Barnes. 1973. Forrest ecology. 2d ed. Ronald, New York. 
Comprehensive ~ covers forests from genetics through physical factors, ecosystem 
characteristics, and historical development. 

Strahler, A. N., and A. H. Strahler. 1973. Environmental geo science. Hamilton, Santa Barbara, 
Calif. Contains a good introduction to soils and the hydrologic cycle. 

Study of Man's Impact on Climate (SMIC). 1971. Inadvertent climate modification. M.I.T. Press, 
Cambridge, Mass. Contains estimates of flows in global hydrologic cycle. 

University of California Food Task Force. 1974. A hungry world: The challenge to agriculture. 
Division of Agricultural Sciences, University of California, Berkeley. Use of land and water in 
world agriculture. 

U.S. Department of Agriculture. 1974. Foreign agricultural economic report, 298. Government 
Printing Office, Washington, D.C. Useful data on land use in agriculture. 

U.S. Department of Agriculture. 1973. The outlook for timber in the United States. Government 
Printing Office, Washington, D.C. Good data source. 

U.S. Department of Commerce. 1975. Statistical abstract of the United States. Government 
Printing Office, Washington, D.C. Data on the uses and supplies of land and water. 

U.S. Water Resources Council. 1968. The nation's water resources. Government Printing Office, 
Washington, D.C. Standard reference on water supply and use, now somewhat dated. 

Vink, A. P. A. 1975. Land use in advancing agriculture. Springer-Verlag, New York. A detailed, 
technical discussion. 

Wollman, N., and G. Bonem. 1971. The outlook for water. Johns Hopkins, Baltimore. Contains 
useful treatment of water quality and waste treatment. 

Wood, N. 1971. Clearcut. Sierra Club, San Francisco. Survey of questionable practices in the 
timber industry, with emphasis on the West. 

Worman, S. 1975. "Agriculture in China". Scientific American, June, pp. 13-21. Useful survey of 
a subject on which little information is available. 

Young, Gale. 1970. "Dry lands and desalted water". Science, vol. 167, pp. 339-343 (January 23). 
Expresses hope of desalting for large-scale agriculture, based on availability of cheap energy. 


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A Hungry World 

The human brain, so frail, so perishable, so full of inexhaustible dreams and hungers, burns by 
the power of the leaf. 

— Loren Eiseley, The Unexpected Universe, 1969 

Photographs of Earth taken from the moon make the finite nature of our planet apparent in a way 
that no writing can. But knowing a vehicle is finite and knowing how many passengers it can 
carry are not the same thing. What is the actual capacity of Earth to support people? 
Unfortunately, there is no simple answer to this question, although certain theoretical limits may 
be calculated. Justus von Liebig's principle, known as the "law of the minimum," says, in 
essence, that the size of a population or the life of an individual will be limited by whatever 
requisite of life is in the shortest supply. It is not yet entirely clear what that requisite will be for 
the human population, which, as we have seen, is growing at an extraordinary rate. But the 
likeliest factor to limit Earth's capacity to support Homo sapiens is the supply of food, since this 

supply depends on the availability of so many other essential resources: land, water, nutrients, 
and energy. 

Apart from the limits that may ultimately be posed by Earth's absolute capacity to support human 
beings, there are gross differences between groups of people with regard to their food supplies. 
The rich are abundantly, even wastefully fed; the poorest live perpetually on the brink of famine. 

This chapter describes the present nutritional status of the human population and the outlook for 
meeting future 

-283- PAGENUMBER284 

• * 




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FIGURE 7-1 A. An example of slash-and-burn agriculture. The new clearing in second-growth 

forest contains many stumps of trees that have been cut high for use as props for growing plants. 

Some, although stripped of their leaves, will survive; along with invading tree seedlings they will 

slowly reforest the garden site. (Photo by Ray A. Rappaport.) B. An example of modern 

mechanized agriculture, which is characterized by high-energy inputs to maintain stability and 

high yields. These machines are wheat combines. (Photo by William A. Garnett.) 

needs for food. It explores how the prognosis might be improved: ways in which food production 
might be increased, and how patterns of distribution might be changed to provide more food for 
the hungry millions. First, however, some description of the elements of food production is in 


At the most basic level of all, human beings, like all other animals, have always been dependent 
on the process of photosynthesis for food. Whether primitive people ate berries, roots, fishes, 
reindeer, or whatever, the energy derived from food had the same ultimate source: the radiant 
energy of the sun. 

Not until the agricultural revolution, however, did humanity begin to exercise some control over 
plant growth and attempt to concentrate and increase the yield from desirable food plants. {Yield 
refers to production per unit of land area, such as an acre or a hectare.) The earliest attempts at 
agriculture doubtless were based on the astute observation that certain accidental disturbances of 
the land by human activities increased the growth of some useful plants. Indeed, the "slash- 
andburn" agriculture (milpa agriculture) practiced today in many humid tropical areas consists of 
little more than cutting and burning clearings in which seeds or cuttings of various desirable 
plants are then scattered. It would have been a small step from such a practice to the reduction of 
competition for the desired plants by simply hoeing weeds, protecting the crop from animals, and 
making use of the fertilizing effects of excreta and other organic wastes. i 

Modern agriculture as practiced in most developed nations is, of course, completely different 
from milpa agriculture (see Figure 7-1 ) . In many respects it is quite different even from 
traditional temperate-zone agriculture as still practiced in parts of Europe and many less 

Charles B. Heiser, Jr., Seed to civilization: The story of man's food, W.H. Freeman and 
Company, San Francisco, 1975; Jack R. Harlan, The plants and animals that nourish man. For 
some interesting ideas on the origin of milpa agriculture, see David R. Harris, The origins of 
agriculture in the tropics, American Scientist, vol. 60, pp. 180-193. It is possible that such 
cultivation, using cuttings rather than seeds, began in the tropical areas of Africa and South 
America as early as, or even earlier than, the first grain cultivation in Asia and Central 


An orange grove in Southern California being replaced by a subdivision. At lower left are 

undisturbed orange groves; at upper center is finished housing. At right is housing in various 

stages of completion; at lower right is a freshly cleared area with one orange tree remaining per 

lot. Since the photograph was taken (in the 1950s), the entire area has been developed. (Photo by 

William A. Garnett.) 

developed countries outside the tropics. The changes in temperate agriculture during the past few 
hundred years could quite fairly be considered a second agricultural revolution. The science of 
plant breeding has produced new crop varieties that are adapted to various growing conditions, 
high in yield, resistant to diseases, and so forth. Mechanical cultivation and harvesting, improved 
methods of fertilization and irrigation, the use of chemical and biological controls against plant 
and insect pests, weather forecasting, and many other technological advances have greatly 
increased the quantity of food that can be produced on a given area of land. 

Some of these advances are mixed blessings, however. Many have unwelcome environmental 
side-effects, which will be discussed later in this chapter and further in Chapter 1 1 . Some 
methods may contribute to higher productivity, but, if they are not used with care, their 
contribution is made at the expense of future production as soil depletion is accelerated. 

Technology has also increased the quality of many crops, but not necessarily in all respects. For 
instance, high yields in grains have sometimes been gained at the cost of lowered pest resistance 
or other undesirable changes in characteristics. In general, high-yielding crops require 

considerably more support and attention than traditional strains in order to realize their full 
potential productivity. 

Although human action can modify many of the conditions of plant growth, limits are imposed 
upon agricultural production by geographic variation in the amount of solar energy reaching the 
surface of Earth, temperatures of both soil and air, amount of soil moisture available, and so 
forth. And, because of the key role played by photosynthesis in agriculture, it is inevitable that 
farming will remain a highly dispersed human activity. Agriculture must remain spread over the 
face of Earth, because the energy of sunlight can only be utilized in photosynthesis at its point of 
arrival. Furthermore, especially when populations are large, agricultural production and the 
transport of agricultural goods will always remain intimately intertwined; food production cannot 
be considered in isolation from food distribution. It is not possible to concentrate agriculture in 
regions of need, as it is sometimes possible to concentrate production of other substances 
required by human beings. 

Indeed, high and growing concentrations of human beings tend to be inimical to agriculture; food 
must be brought to them, and they occupy land that could produce it. As anyone knows who has 
lived in the country around Philadelphia, Chicago, or Los Angeles, for instance, farmland around 
cities is disappearing beneath pavement-each year about 250,000 hectares of prime agricultural 
land in the United States are "developed" into subdivisions and highways. _Between the early 
1950s and early 1970s, some 1 1 million hectares were converted into airports, freeways, and 
suburbs—a total area larger than Ohio. _If less than first-class land is considered, the quantity 
that has been—and continues to be— lost rises to over 18 million hectares since 1945, with an 
annual loss of a million hectares in the early 1970s. — 

In recent decades, for each 1000 people added to the population of California, an average of 100 
hectares of arable land has been covered by buildings and pavement. 4 Figure 7-2 graphically 
shows the process underway. Residents and industry also compete for freshwater 

2 Time, "The world food crisis", November 11, 1974. 

D. Pimentel, et al., Energy and land constraints in food protein production. 
a Pimentel, et al., Land degradation: effects on food and energy resources. 

K. E. F. Watt, personal communication. 

supplies, which are becoming increasingly scarce in many regions. (See Chapter 6 for more on 
conversion of land to urban uses and dwindling untapped water supplies.) Moreover, smog can 
reduce yields and even kill crops. 

If farmland continued to be converted to urban use at the rates prevailing in the 1960s, California 
eventually would not be able to feed itself, let alone export food to other states and abroad as it 
does today. In dollar volume, California is the most important agricultural state. Fortunately, 
since 1970 immigration into California has slowed to a trickle, and rising farmland values, new 
protective tax laws, and actions of environmental groups have substantially reduced the rate of 
farmland conversion. 

In general, it is important to remember that the agricultural system of a nation is an integral part 
of its socio-economic system, and that this often places constraints on how much food is 
produced. ^_For example, it is often overlooked in discussions of the world food problem that 
farmers, like plumbers, corporate executives, and university professors, expect economic rewards 
for their labors. They will not grow food to sell for less than it costs to grow it. Thus the 
economics of agroecosystems inevitably pervades realistic discussions of how much food can be 
grown in various places and how much can be stored and distributed in various patterns. 

Major Food Crops 

Some 80 species of food plants have been domesticated, as opposed to only about two dozen 
kinds of animals. Over the centuries, most of them have been improved by selective breeding 
and/or by hybridization. Virtually all the original domestication of both plants and animals took 
place in prehistoric times. ^But, in spite of the diversity of available food plants, a relatively 
small array of crops supplies the great bulk of the world's food. If one had to pick the three most 
important food plants in the world, the almost inevitable choice would be three species of 
grasses: rice, wheat, and corn. So important are these cereal grains that slightly more than one- 
half of the 

TABLE 7-1 Sources of Humanity's Food Energy 

Percentage of 
Food energy supplied 


Rice 21 

Wheat 20 

Corn 5 

Other cereals 1 


Potatoes and yams 5 

Cassava 2 





total 100 

Source: Lester R. Brown and Erik P. Eckholm, By bread 

harvested land of the world is used to grow them. Wheat and rice together supply roughly 40 
percent of humanity's food energy ( Table 7-1 ). Altogether, cereal grains provide well over half 
of the calories consumed by the human population and nearly half of the protein. - 

Rice is the most important crop of all; it is the staple food for over two billion people. As is 
shown in Figure 7-3 , the People's Republic of China grows about 35 percent of the world's rice; 
India, Bangladesh, Indonesia, Japan, and Thailand account for another 43 percent. Most of the 


remaining 22 percent is grown in Southeast Asia and Latin America. _Total world production of 
rice (before milling) in 1974 was an estimated 323 million metric tons. 

Wheat almost equals rice in importance to the diet of human beings. (Sometimes wheat 
production in tonnage exceeds that of rice, but some of it is used as feed for livestock.) In 1974 
some 360 million metric tons of wheat were produced. Unlike rice, wheat does not grow well in 
the tropics, in part because one of its major diseases, wheat-rust fungus, thrives in warm, humid 
climates. Wheat is grown mostly where winters are cold and wet, 

Robert S. Loomis, Agricultural systems. 
Stuart Struever, ed., Prehistoric agriculture. 
Pimentel, et al., Energy and land constraints. 

8 United Nations Food and Agriculture Organization, Production yearbook, 1974 ( FAO, 
Rome, 1975); J. Janick et al., Plant science. These two are the main sources for what follows. 



I 3 f 9 


















Where major crops are grown. (Data from FAO, Production yearbook, 1973.) 

and summers hot and rather dry (although in the U.S. wheat belt most of the precipitation occurs 
in the summer). The Soviet Union produces nearly 30 percent of the world's wheat. Most of the 
rest is grown in such temperate areas as China, northern India, North America, Europe, and 

Corn, or maize, is the third great cereal crop, with 1974 production being about 293 million 
metric tons. The long, warm, moist summers of the eastern half of the United States are ideal for 
corn production, and more than 45 percent of the world supply is grown there. It should be noted, 
though, that the bulk of American corn production is fed to livestock; human consumption of 
corn as a principal staple is mainly confined to Latin America and tropical Africa. 

Rice, wheat, and corn together accounted for nearly one billion metric tons of grain in 1974. The 
rest of the world's cereal grain crop of about 1.2 billion metric tons was made up by other grains: 
barley, oats, rye, millet, and sorghum. Somewhat more than half the world's production of these 
grains come from the United States, the USSR, and Western Europe. These grains, together with 
corn, are commonly referred to as "coarse grains" (as contrasted with wheat and rice, the "small 


Nitrogen-fixing bacteria inhabit the nodules of soybean roots in a mutualistic relationship. (Photo 

by J. C. Allen and Son.) 

Roughly 300 million metric tons of potatoes are also grown annually, but the water content of 
the potato is so high (75 percent) that the food value of the total harvest is considerably less than 
that of any of the "big three" grains. (On a per-acre basis, however, the potato may be superior.) 
The potato is particularly well-adapted to cool climates. Fully one-third of the world's potato 
production comes from the Soviet Union, 16 percent comes from Poland, and over 1 1 percent 
from China. The United States and East and West Germany are also important producers. The 
rest of the world accounts for less than 30 percent, mostly European and Latin American 

The protein content of most modern grains ranges between 5 and 15 percent and is not complete 
protein (that is, it does not have an ideal balance of amino acids for human nutrition). In most 
grains the protein is too low in content of some essential amino acids, especially tryptophan and 
lysine. Grains and potatoes are all rich in protein, however, in comparison with some of the 
starchier staple crops commonly consumed in humid tropical countries: taro, cassava, yarns, and 

Legumes (a family of plants that includes peas, beans, peanuts, soybeans and several forage 
crops) cannot compete in volume with grains in world food production. They have two to four 
times the protein content of grains, however, and thus are critically important in human nutrition 
and for domestic animals as well. For the human population, legumes provide about 20 percent 
of the protein supply worldwide. ^Bacteria associated with the roots of legumes have the ability 
to fix gaseous nitrogen from the atmosphere and convert it into a form that can be directly used 
by plants (See Figure 74 ). As a result, legumes also serve as fertilizer ("green manure") when 
planted in rotation or intercropped with other crops, and thereby also can contribute indirectly to 
the protein derived from other plants. 

Two legumes, soybeans and peanuts (groundnuts), are grown mainly for use as oil sources and 
livestock feed. These two crops account for about two-thirds of the world's legume production of 
some 124 million metric tons; soybeans alone reached 63 mmt in 1973. _^JSoth are excellent 
sources of protein, especially soybeans. The protein content of soybeans is some three times as 
high as that of wheat, for instance. Both soybean oil and peanut oil are used for making 
margarine, salad dressings, and shortenings, and are also used in various industrial processes. 
The high-protein material remaining after the oils are pressed out (presscake) is usually sold for 
livestock feed. Except in the Far East, relatively little of the soybean crop is directly consumed 
by people, although this is changing as inflation induces people in DCs to substitute cheaper 
foods for meat. Soy products are becoming increasingly popular with Americans. A significant 
portion of the peanut crop has long been eaten in the nut form or in candy or peanut butter 
around the world. Production of soybeans is concentrated in the United States and China, 
although production in Brazil was increasing rapidly in the 1970s. Peanuts are grown mainly in 
India and Africa. 

The remaining legumes—beans and peas—are known collectively as pulses. There is a wide 
variety of these, including lima beans, string beans, white beans, kidney beans, scotch beans, 
peas, cow peas, garbanzos, and 

10 Folke Dovring, Soybeans. This is an interesting account of the growth in importance of 
soybeans as a major food crop. 
Pimentel, et al., Energy and land constraints . 
-288- PAGENUMBER289 

lentils. Because they are among the richest plant sources of protein, pulses provide a significant 
element in the human diet, especially in poor countries. Moreover, the proteins of pulses, 
although also incomplete, complement those of grains to furnish a complete protein meal when 
they are eaten together. 

Unfortunately, annual world production of pulses has not increased significantly since 1962. In 
developed countries, they may have been replaced in part by increased consumption of meat and 
dairy products. But in rapidly growing less developed countries, the lack of increase in pulse 
production for more than a decade indicates a considerable reduction in per-capita supplies. 
Since the gap has mainly been filled by grains, the probable result is a deterioration of protein 
levels in diets, both in quantity and quality. In some countries, acreage planted to pulses has 
actually been reduced in favor of more profitable high-yield grains. — 

Cereals and legumes are the worldwide mainstays of the human diet, but a vast variety of other 
plants is cultivated and consumed. Cassava, taro, sweet potatoes, and yams (all root crops) 
supply food energy to many people in the world, especially to the poor in humid tropical areas; 
people subsisting mainly on these starchy foods are relatively likely to suffer from protein 
deficiency. The roots of the sugar-beet plant and the stems of the cane-sugar plant (a grass) 
supply sugar to people around the world. The roots, stems, fruits, berries, and leaves of numerous 
other plants are important sources of energy, vitamins, and minerals in human diets. 

Many plants are also used as forages—foods for domestic animals. Although ruminants (cattle, 
sheep, and goats) are often just turned loose on rangeland to graze and fend for themselves, many 
forage crops are grown in pastures or harvested and used for hay or silage (fermented fodder) 
specifically to feed them. In the United States, which consumes large quantities of meat per 
capita, the forage crop fed to animals in 1973 was only slightly less (as measured by nutritional 
equivalent) than the entire U.S. grain harvest for that year (and much of that was also fed to 
animals). Some 60 million acres of the world are planted in the legume alfalfa (called "lucerne" 
in Europe), which is among the most nutritious of all forage crops and is especially rich in 
protein. Some other legumes—for instance, clovers— are also grown as forage, as are a great 
variety of grasses. — 

Food from Animals 

The primary nutritional importance of domesticated animals in many cultures is as a source of 
high-quality protein. They are, of course, valued for other roles as well, especially in less 
developed countries where traditional techniques of agriculture still prevail. There the larger 
farm animals— cattle, horses, and water buffalo-are highly valued as draft animals and as 
transport. The dung of all livestock is valued everywhere as an important source of fertilizer and 
in some regions as fuel and building plaster. In some societies, the value of animals for food is 
almost incidental to their other uses. 

The variety of animals that have been domesticated for food is considerably more limited than 
that of plants. Only nine species-cattle, pigs, sheep, goats, water buffalo, chickens, ducks, geese, 
and turkeys—account for nearly 100 percent of the world's production of protein from 
domesticated animals. Beef and pork together, in roughly equal amounts, account for some 90 
percent of meat production (excluding poultry). Cows produce more than 90 percent of the milk 
consumed, water buffalo about 4 percent, and goats and sheep the remainder (ignoring tiny 

1 3 

amounts from reindeer and some other minor domestic mammals). — 

Although certain breeds of domestic animals are adapted to the tropics, animal husbandry is 
generally easier and more productive in temperate areas than in the tropics. It is primarily in the 
temperate zones that geneticists have produced animals capable of extraordinary yields of meat, 
milk, and eggs, given special care and intensive feeding of high-protein feedstuffs. The 
yearround high temperatures and possibly the high humidity of the tropics tend to slow growth 
and, in milk-producing animals, lower the production of milk and milk solids. Unfortunately, 
although forage may grow luxuriantly in many tropical areas, the native varieties are commonly 

J. D. Gavan and J. A. Dixon, India: A perspective on the food situation. 
H. J. Hodgson, Forage crops. 

1 3 

IH. H. Cole, ed., Introduction to livestock production; H. H. Cole and M. Ronning, eds., 
Animal agriculture. 

low in nutrient value. High temperature and humidity also often provide ideal conditions for 
parasites and carriers of disease. For instance, in a large portion of Africa where rainfall and 
other conditions are suitable (about 37 percent of the continent) tsetse flies carry a serious 
disease, nagana (caused by single-celled organisms called trypanosomes), which makes cattle 
herding impossible. — 

Domestic animals, especially cattle, are often more than mere meat or milk producers in LDCS. 
In the semiarid zones of East and West Africa, cattle are the basis of entire cultures. They are 
regularly tapped for blood as well as milk, and are intimately related to the social and economic 
life of certain groups. Cattle provide their owners with wealth and prestige, are used 
ceremonially, and have aesthetic value. 

In India there is a large population of "sacred cattle," so called because of the Hindu taboo 
against slaughter. Visitors to India rather commonly conclude that the Indian food situtation 
could be ameliorated by slaughtering these "useless" animals. This judgment is based on a 
fundamental misunderstanding of the situation. Like so many folkways and taboos, the Indian 
taboo has a vital influence on the local ecology. Most of the cattle feed on forage and waste 
vegetation that are not human foods: they do not compete with people for food. The cattle do 
supply milk, and above all they supply power. Bullocks (castrated bulls) are the tractors of India; 
they are absolutely essential to her agricultural economy. Finally, the cattle also supply dung, 
which serves as the main cooking fuel of India, and which is also used as fertilizer and as plaster 
in houses. Of an estimated 800 million tons of dung produced each year, some 300 million were 

used as fuel in the 1960s. This fuel produced heat equivalent to that obtained from burning 35 
million tons of coal (about half of India's coal production). — 

The consumption of meat and poultry has grown very rapidly since World War II, especially in 
developed countries. World production in 1974 amounted to just under 1 16 million metric tons, 
of which the developed regions produced about 79 million metric tons and the less developed 
regions produced the remaining 37 million metric tons. 16 Meat clearly occupies a far less 
important position in the diets of people in LDCS, with the notable exception of herding societies 
such as the Masai, the people of the Sahel, and nomads in the Middle East and Central Asia. 


In 1967 the President's Science Advisory Committee Panel on the World Food Supply — 
estimated that 20 percent of the people in the less developed countries (which include two-thirds 
of the world's population) were undernourished (that is, were not receiving enough calories per 
day) and that perhaps 60 percent were malnourished (seriously lacking in one or more essential 
nutrients, most commonly protein). As many as a billion and a half people thus were described as 
either undernourished or malnourished. This may have been a conservative estimate; others have 

1 R 

placed the number of "hungry" people during the mid-1960s at two billion or more. The 

President's Panel further estimated that perhaps a half billion people were either chronically 
hungry or starving. (These numbers did not include the hungry and malnourished millions in the 
lower economic strata of developed countries such as the United States, or the numbers of people 
who could afford to eat well but were malnourished because of their ignorance of elementary 
nutrition.) Nor did this situation suddenly develop in the 1960s; chronic hunger had long existed 
among the poor around the world. 

Recent History of Food Production 

Between World War II and 1972, there was a steady, worldwide, upward trend in the amount of 
food produced for each person on Earth. This trend generally continued uninterrupted in the 
developed countries, with a few exceptions and setbacks, until the 1970s. In the less developed 
regions, however, the steady increase 

D. F. Owen, Animal ecology in tropical Africa. 

A. Leeds and A. P. Vayda, eds., Man, culture, and animals. 

F AO, Production yearbook, 197 '4. 

1 7 

The world food problem vol. 2, p. 5. 
18 G. Borgstrom, Too many. 
-290- PAGENUMBER291 

TABLE 7-2 Estimated Number of People with Insufficient Protein/Energy Supply by 
Regions (1970) 

Percentage Number below 
Population below lower lower limit 

Region (in billions) limit (in millions) 

(in billions) 


below lower 


Number below 

lower limit 

(in millions) 


















Developed regions 

Developing regions excluding Asian 

centrally planned economies 

Latin America 

Far East 

Near East 


World (excluding Asian centrally 

planned economies) 2.83 16 462 

Source: UN, Assessment. 

in per-capita food production was halted sometime between 1956 and 1960, depending on the 
country. Because population growth in LDCs more or less equalled the growth in total food 
production, the available supply per person has fluctuated around the same level ever since. In 
1970 the average country in Africa was producing less food per person than it had in the mid- 
1950s, while the average country in Latin America and the Far East had slightly improved its 
condition. Among less developed regions, only China and the near East significantly increased 
their food supplies per person between 1955 and 1970. This situation obtained in spite of 
substantial increases (roughly 30-35 percent) in absolute food supplies in all these areas during 
that period; population growth offset the gains. 

There was widespread optimism in the late 1960s and early 1970s that improvements in 
agricultural techniques, especially in less developed nations (the Green Revolution), would allow 
food-production increases—at least for a few decades—to exceed the 2 percent annual growth of 
the world's population, which was then adding about 75 million hungry mouths every year. 
Between 1967 and 1972, food production did indeed rise more rapidly than population growth, 
most encouragingly in some of the hungriest nations in Asia. Even so, a large segment of the 
human population was not receiving sufficient daily bread. 

The United Nations has very conservatively estimated that some 460 million people were 
"seriously undernourished" during those relatively food-abundant years just before 1972. 
Moreover, the UN conceded that, by less conservative criteria, the number of hungry people at 
that time actually might be more than twice as great. Because infants and young children are 
most likely to suffer when food supplies are inadequate, the UN further estimated that perhaps 
one-half of all children underfive years old in developing countries were inadequately nourished 
to some degree. — 

Table 7-2 shows the UN estimates of the distribution of undernourished people in the world by 
region in 1970. It should be noted that even in the generally well-fed developed regions, 3 
percent, or 28 million people, were considered undernourished. Not a few of those were in the 
United States. Figure 7-5 shows where the hungry nations are located. 

Since 1971, the prospects for raising nutritional levels in the poor countries have dimmed. The 
early 1970s, particularly 1972 and 1974, were marked by unusually adverse weather for 
agriculture in a broad array of regions, including densely populated Southern Asia, agriculturally 
important North America, much of tropical Africa, and parts of Southeast Asia, Brazil, the Soviet 

United Nations, Assessment of the world food situation, present and future. The estimate 
excluded China and other centrally planned countries in Asia because of insufficient data; 
China was not admitted to the United Nations until late 1971 . It is worth noting, however, that 
the accumulating evidence, though not at all conclusive, indicates that the people of China 
and the other centrally planned Asian countries ( North Vietnam and North Korea) have been 
fairly adequately fed in recent years. China responded to a poor harvest in 1972 by buying 
wheat from the United States. So far as anyone can tell, the Chinese people suffered no 
significant ill-effects from food shortage then. 

The geography of hunger, 1969-1971 . (Data from Assessment of the world food situation, present 

and future, UN World Food Conference, 1974.) 

Union, and China. These widespread weather changes, unanticipated by the agricultural 
community, manifested themselves mainly as floods, droughts, and sometimes both, at different 
times. The consequence of this series of weather changes, exacerbated in 1974 by the fourfold 
increase in oil prices and a severe shortfall in fertilizer production, was an unprecedented 
worldwide shortage of food by the end of that year. 

In 1972, after a strong upward trend for 20 years, world food production declined. Production of 
grains-wheat, rice, and coarse grains—on which people in poor countries primarily depend for 
sustenance, fell 33 million metric tons below 1971 harvests, about 3 percent. To meet the 
growing demand, 25 million metric tons more than the 1971 total were needed. At that time 
adequate reserves existed; so most countries were able to obtain enough grain on the world 
market or from food aid to meet their needs, at least at prevailing inadequate standards. 

The primary consequence of the 1972 shortfall in production was a worldwide steep rise in 
prices of basic foods, especially of grains. Higher food prices inevitably most affect the poorest 
people in every nation. The blame for the price-rise in grains has been attributed to speculation 
on the world market and to unusually large grain purchases in 1972 and 1973 by the USSR and 
China—some 30 million metric tons— mostly from the United States at low, government- 
subsidized prices. The basic cause, of course, was the production shortfall (and subsequently 
diminishing reserves), but this has often 



been overlooked, at least in the popular press. Between 1972 and 1974, world market prices for 
wheat, rice, corn (maize), and soybeans tripled or quadrupled. Later they declined somewhat, but 
generally have remained at about twice their 1972 levels. — 

There was a partial recovery of food production in 1973, mainly in DCs, and per-capita grain 
production returned to "normal" levels on a worldwide average. Grain reserves continued to 
decline, however, as they were drawn down to compensate for the 1972 deficit. 

Widespread bad weather struck again in 1974, coinciding with the energy crisis and a severe 
fertilizer shortage. This time the decline over the 1973 grain production amounted to some 50 
million metric tons -even more than the decline of 1972, a drop of more than 4 percent. Per 
capita, of course, the decline was even greater: about 6 percent. 

The world food picture in late 1974 was considerably gloomier than the unattractive situation 
described in detail by the President's Panel on the World Food Supply in 1967. World grain 
reserves (stored grain, mostly U.S. surplus, plus the potential production of idled land) in 1967 
were equal to 55 days' supply, the low point for that decade. In 1974 reserves were down to a 33- 

9 1 

day supply — essentially what exists in the "pipeline" of supply channels. — 


UN, Assessment; USDA Economic Research Service, World agricultural situation, WAS- 12, 
December 1976. 

9 1 

Lester R. Brown, The politics and responsibility of the North American breadbasket. 
-293- PAGENUMBER294 

Large-scale famine was then occurring in the Sahel (the southern fringe of the Sahara desert, 
including parts of the nations of Chad, Gambia, Mali, Mauritania, Senegal, Upper Volta, and 


Niger), where drought had prevailed for six consecutive years. Famine was also threatening 


India and Bangladesh, both of which had been struck by floods and at least partial monsoon 

failures in two of the previous three years. Other regions threatened with severe food shortages 
included some thirteen additional African countries, comprising virtually all of tropical Africa; 
four Central American and Caribbean nations; Pakistan, Sri Lanka, Laos, and Cambodia in Asia; 
and Yemen and South Yemen in the Middle East. 

In October 1974, the United Nations' Food and Agriculture Organization (generally known as 
FAO) announced that no less than 32 nations, within which nearly three-quarters of a billion 
people were already marginally or inadequately nourished, were threatened with starvation and 
bankruptcy unless developed nations provided them with food and economic assistance. 

On top of the widespread adverse weather in 1974, the oil embargo of fall 1973, followed by the 
quadrupling of oil prices by OPEC nations, caused serious problems for agriculture. Apart from 
the need for gasoline to operate farm machinery, petroleum and (especially) natural gas are 
needed for the manufacture of fertilizers. Manufactured fertilizers were already in short supply; 
the effect of the oil crisis was to worsen the shortage of fertilizers worldwide. And less 
developed countries, which have become increasingly dependent on fertilizers to raise their food 
production, were most hard hit. The UN estimated that the fertilizer shortage might have reduced 
grain production in LDCs in 1974 by 12 million metric tons. — 

Almost simultaneously with the drops in world agricultural production, fisheries harvests, which 
had also risen steadily since World War II, began to decline. The peak, over 70 million metric 

tons offish, was reached in 1971. In 1972 and 1973, the fish catch fell below 66 million metric 
tons. ^Although fisheries production contributes only a small percentage of the calories in the 
world food budget, it is a vital source of protein, especially to protein-short LDCS. 

Hopes were high that 1975 harvests would brighten the world food picture and allow some 
rebuilding of grain reserves. But total production barely exceeded the disastrous 1974 level. This 
time the poor harvests were centered mainly in the developed countries, especially the USSR. 
Because of bad weather again, the 1975 Soviet grain harvest fell some 55 million metric tons 
below that of the previous year. Meanwhile, the less developed nations enjoyed an average 5 

9 7 

percent increase in food production in 1975 after three consecutive poor years. — 

The total world cereal crop for 1975/ 1976 was estimated by the U.S. Department of Agriculture 
at 1220 million metric tons, only 20 mmt above the previous year and still 34 mmt below the 
record of 1973/ 1974. Since most of the 1975/ 1976 shortfall occurred in DCs (especially the 
USSR), those countries absorbed it, largely through reduced meat consumption. Nevertheless, 
averaged worldwide, production of grain per capita, at 309 kilograms of grain per person, 
reached a ten-year low, and there was no improvement in the carryover supply. 

The picture brightened somewhat in 1976/ 1977, when preliminary reports indicated a total grain 
harvest of over 1320 mmt. 27a This increase of 100 mmt was achieved despite severe floods in 
Japan and parts of southeast Asia, drought in the north-central United States, and a drought in 
Western Europe unequalled in many centuries. Grain reserves, mostly of wheat, rose 


Numerous popular articles have appeared in recent years on the Sahel tragedy. Among the 
more recent are: "The drought revisited", Time, April 21, 1975; "Sub-Sahara land hopeful on 
crops", New York Times, November 10, 1974, "Disaster in the Sahel", New Internationalist, 
no. 16, June 1974. The ecological basis of the famine is discussed in Chapter 11. 
"Vivid descriptions of the situation in India by fall of 1974 can be found in: Letter from New 
Delhi", New Yorker, October 14, 1974; "India's grim famine: With luck only 30m will starve", 
The National Times (Australia), October 28-November 2, 1974; "India: An unprecedented 
national crisis", by Marcus Franda, Common Ground, vol. 1, no. 1, pp. 63-76 ( American 
Univ. Fieldstaff), Jan 1975; "Anguish of the hungry spreading across India", by Bernard 
Weinraub, New York Times, October 27, 1974. 


Kasturi Rangan, "Bangladesh is faced with large-scale deaths from starvation", New York 
Times, October 11, 1974; Kasturi Rangan, "Bangladesh fears thousands maybe dead as 
famine spreads", New York Times, November 13, 1974; Kushwant Singh, "Bangladesh, the 
international basket case", New York Times Magazine, January 26, 1975. 

25 United Nations, Assessment. 

26 United Nations, Statistical yearbook, 1974. 

27 USDA, WAS-9, WAS-10, and WAS-12. 

27a USDA, WAS-12. 

in 1977 for the first time since 1972 but were still far short of goals set by the 1974 World Food 

The outlook for 1977/ 1978 was not encouraging. The disastrous winter of 1976/ 1977 in the 
United States alone, with record cold and snowfall in the East and severe drought in the West, 
could not fail to reduce agricultural production in the American granary. Once again, plans to 
build up world grain reserves as insurance against weather-caused reductions in grain production 
may be defeated by that same eventuality. 

Agriculture's continuing dependence on weather thus has been emphatically brought home by the 
events of the 1970s. Moreover, the damage has not been limited to LDCS; such supposedly 
secure granaries as the United States, Europe, Australia, and the USSR have also suffered. Table 
7-3 shows the U.S. Department of Agriculture estimates for the world's total grain production 
during the 1970s. Table 74 shows the decline in annual levels of the world's grain reserves since 

Figure 7-6 shows the general trends of per-capita food production in developed and less 
developed areas (excluding the People's Republic of China) since 1955. Note that, like total 
production, per-capita food production in DCs has risen steadily, but in LDCs in 1975 it had 
barely recovered to the peak level achieved in 1970, not far above the level of the period 1961- 
1965. This, of course, indicates an enormous increase in human misery. Well over a billion 
people have been added to the populations of less developed nations since the early 1950s, and a 
large proportion of them are both very poor and chronically undernourished. Even though the 
average nutritional condition of the poverty-stricken of our planet may have remained about the 
same during those decades, both the absolute numbers of the poor and their proportion of the 
total world population have increased rapidly. 

The worldwide nutritional situation that has characterized the past generation is totally 
unprecedented, in part because of the absolute numbers of people involved. Famines have 
occurred throughout history, but they have generally been temporary, if cataclysmic, events 
caused by weather or human conflicts and limited to relatively small, local populations. Such 
famines were undeniably tragic affairs that resulted in a great deal of human suffering, and 
chronic hunger has also been long 

TABLE 7-3 

World Grain Production, 

1969-1970 Through 1976-1977 - 


1969/ 1970-1971/ 1972, averaged 

1972/ 1973 

1973/ 1974 

1974/ 1975 

1975/ 1976 

1976/ 1977 


Grain production 
(million metric tons) 


Wheat, milled rice, and major coarse grains (corn, barley, rye, 
oats, and sorghum). 


Source: USDA, World agricultural situation, WAS-12, December 

1976, p. 18.1972/1973 data from WAS-9, December 1975, p. 36. 

TABLE 7-4 

Index of World Food Security, 1961-1976 

(figures are in million metric tons) 


Grain production 
(million metric tons) 





of grain 



of idled 





Reserves as 

days of 
annual grain 
























































Based on carry-over stocks of grain at beginning of crop year in individual 
countries for year shown. 

Preliminary estimates. 

Source: Lester R. Brown, The politics and responsibility of the North American 

breadbasket. Worldwatch Institute. 











120 - 

80 h 







120 h 


120 - 



{V^J \ 











i> A 




80 7 

120 * 
'00 ° 


-1 120 




105* 1970 

FIGURE 7-6 World food production per capita, 1954-1975. The developing countries have 

gained only 0.4 percent per year. In none of those regions has the index reached 110, and in 

Africa it has shown a downward trend since 1951 . Per-capita food production trended upward 

1.5 percent per year in the developed countries until the early 1970s. In each of those regions the 

index of per-capita food production has reached or exceeded 1 10 at least three times in the 22- 

year period. (Adapted from U.S. Department of Agriculture, The world food situation and 
prospects to 1985, Foreign Agricultural Economic Report No. 98, 1974; recent data from WAS- 


endemic in many areas. But the scale and ubiquity of hunger today is an entirely new 
phenomenon. Unceasing poverty-related privation, including hunger and malnutrition, is now 
endured by more than one billion people around the globe, and perhaps 400 million live on the 

brink of starvation. This chronic hunger is also unprecedented because the multitudinous 

hungry have become increasingly aware of the dietary condition of the affluent few and have 
high hopes of emulating them, a situation that has grave political implications for the future. 

This, then, is the fundamental reason that the human population reached such a precarious 
position with regard to its most basic resource — food — so suddenly in the mid-1970s: an 
already marginal situation was precipitated into a crisis by a series of weather-related bad 
harvests. The roots of the crisis lie: (1) in rapid population growth, which has necessitated an 
increase in food production of about 2 percent per year for the past generation just to maintain 
existing nutritional levels; (2) in the finite amounts of arable land, fresh water, and fertilizer 
available; (3) in the low productivity of traditional farming methods, especially in LDCS; (4) in 
environmental constraints; (5) in the lack of world investment (either monetary, social, or 
political) in increasing food production; and (6) in a political decision in the U.S. to eliminate 
government-held grain reserves. Underlying all the other factors, the world's economic system 
has created and perpetuates gross inequities in the distribution of food, as of other commodities. 
Thus a quarter of the human population have access to abundant food, while nearly half subsist 
on inadequate supplies and are tragically vulnerable to any disruption in that meager allotment. 

Following the Rome Food Conference in November 1974, efforts were made to establish a 
worldwide foodinformation network, and to organize national and international food-reserve 
stockpiles to meet future deficits and to avert famine in the hungriest nations. A World Food 
Council, responsible to the UN Secretariat, was established to foster agricultural development in 
LDCs and to coordinate famine-relief efforts. A Consultative 


Jean Mayer, The dimensions of human hunger. 








| WWW Foua CtHrat' ' 


international Fund 
tar AgnciiflUf Jl (Hjyieloismwl 




Committee cm 
World food Bkmhv 


EMfp HHW4 





Group on Food 


mi mi— rt to 

OrMtopng Countrni 

Cmsulrsliun $roup 
on innimlkmai 



10 Autumn CwKh* 

FIGURE 7-7 Major international organizations in the food field. 1 Members of the World Food 

Council: Argentina, Australia, Bangladesh, Canada, Chad, Colombia, Cuba, Egypt, Federal 

Republic of Germany, France, Gabon, Guatemala, Guinea, Hungary, India, Indonesia ,Iran, Iraq, 

Italy, Japan, Kenya, Libyan Arab Republic, Mali, Mexico, Pakistan, Rumania, Sri Lanka 

(Ceylon), Sweden, Togo, Trinidad and Tobago, United Kingdom, United States, USSR, 

Venezuela, Yugoslavia, and Zambia. 2This loosely organized but important London-based 

consultative mechanism — which is outside the UN system of organizations and therefore has no 

official responsibility for implementing the recommendations of the World Food Conference — 

now involves some sixty countries, including the Soviet Union. It is the body within which 

international wheat agreements have been negotiated since the late 1940s. 3The Soviet Union is 

not a member of the FAO. 4Established by the FAO council in late November 1975 as a 

successor to the FAO Ad Hoc Working Party on World Food Security. (From McLaughlin, 

1975, Overseas Development Council.) 

Group on Food Production and Investment, connected with the World Bank, was set up to 
channel assistance funds into useful agricultural projects. Figure 7-7 shows the major 
international agencies involved with food, inside and outside the United Nations. 

Thanks to a decline in demand for food in the rich countries, especially for feed grains, enough 
food was made available on the world food market and as food aid to head off large-scale famine 
in 1975. Some 36 million tons of grain were imported in 1973/ 1974 by LDCs (not including 

China), and 8.6 million tons were given to the neediest countries during 1974/ 1975. The 

World Food Council has sought 10 million tons of food aid per year, but only about 9 million 

tons were donated in 1975/ 1976. Importations of food by DCs declined to about 25 mmt in 

1976/ 1977, the lowest in six years, thanks to good harvests that year. 30a Nonetheless, while 
grain reserves remain at critically low levels, any significant problems in food production in 
1977 and subsequent years are bound to have serious social, economic, and political 
repercussions. The possibility of a major famine that could bring population growth to a sudden, 

■2 i 

catastrophic halt has never been more real. — 


Per-capita production figures are at best crude indicators of the food situation. They conceal 
gross differences that exist between countries, especially between the developed nations and the 
less developed nations. It is impossible to judge directly from per-capita foodproduction figures 
exactly what is happening to the average diet of individuals within each country, because the 
averaging process also conceals a great range of dietary differences between regions and income 
levels. The wealthiest urban classes in many LDCs eat very well, whereas the poorest — landless 
farm workers or urban shanty dwellers — live from hand to mouth. The prevailing economic 
system encourages production of food for the rich at the expense of food for the poor. — 

Martin M. McLaughlin, World food insecurity: Has anything happened since Rome? 
Jon McLin, Remember the World Food Conference? 
30a USDA, WAS-12. 

1 1 

Lester R. Brown, The world food prospect. 


Michael Lipton, Urban bias and food policy in poor countries. 
-297- PAGENUMBER298 

Regional differences in diet may reflect different economic conditions or they may be the 
temporary result of good or bad harvests. For instance, in some areas of northern India, following 
the famine of the mid-1960s, which had necessitated the importation of nine million tons of grain 
from the U.S., wheat and rice were superabundant, and prices were dropping steadily. Yet, in 
spite of this surplus of food, seven million people were in danger of starvation in one province 
alone, and in northern India as a whole some 20 million people were described as being in "acute 
distress." This situation was due largely to local droughts in areas adjacent to those producing 
surpluses. The people were so poor that they could not create effective "demand" for food, 
producing the spectacle of surpluses and dropping prices in close proximity to starvation. 

Even for an entire country, figures on food production may be unrevealing, since consumption 
equals production plus or minus trade. The trade positions of many less developed countries 
changed markedly between 1956 and 1975; many former grain exporters became heavy grain 
importers. At the same time, many of these hungry nations export food — including such high- 

1 1 

protein foodstuffs as oilseed cakes and fishmeal — to the rich countries. — 

The present failures of food distribution are the result of a host of interacting factors, including 
poverty, ignorance, cultural and economic patterns, and lack of transport systems. Although 
worldwide supplies of food today might theoretically be adequate to feed every human being if 
fairly distributed, the average diet within more than half of the less developed countries is below 
minimum standards of nutrition in calories alone. Distribution of specific nutrients, especially 
protein, is equally, if not more, inequitable, although less well documented. 

Between the early 1950s and 1970s, food production in both developed and undeveloped regions 
increased at approximately the same average rate: around 3 percent per year. The difference is in 
the rates of population growth: less than 1 .5 percent per year for DCs; 2 percent or more for most 
LDCS. Table 7-5 compares rates of population growth with growth in food production between 
1952 and 1972. In LDCS, food production in 1952 was inadequate to feed the populations then 
existing. Since then, there has been a slight improvement overall, but in 34 out of 86 LDCs for 
which the United Nations had sufficient information, food production actually fell behind. If 
rising demand for food, which would be expected to accompany rising incomes, is considered, 
some 53 of these countries were failing to meet their food needs in 1972. ^_In the developed 
world, by contrast, most populations were reasonably well-fed in 1952. Since population growth 
was much slower, increased affluence accounted for more than half the growth in food 
consumption between 1952 and 1972. Thus, whereas people in most non-communist poor 
countries were not significantly better- fed after twenty years of efforts to raise food production — 
indeed, many had lost ground — those in rich countries generally were enjoying much more 
sumptuous diets. — 

Levels of Nutrition 

Some appreciation of the nutritional gap between rich and poor nations can be gained by 
comparing the availability of food energy and protein per capita. According to the United 

Nations Food and Agriculture Organization, some 3100 kilocalories (kcal) per day on the 
average were available to each person in developed countries in 1969-1971, some 23 percent 
above estimated needs. And an average of over 96 grams of protein were available per capita in 
DCs, also well above requirements. By contrast, in LDCs on the average only about 2200 kcal 
and less than 58 grams of protein per capita were available, about two-thirds of the amounts 
available in DCs and 5 percent below estimated needs. Table 7-6 shows the estimated average 
food energy and protein supplies in 1969-1971 in different regions. A more detailed table 
comparing population growth rates and per-capita food supplies for individual countries in 1969- 
1971 can be found in Appendix 2. 

Although individual needs for calories vary according to age, sex, body size, and activity, the 
FAO has 

UN, Assessment. 


See E. Eckholm and F. Record, The two faces of malnutrition, for a good overview of the 
world nutritional situation. 
UN, Assessment. 

TABLE 7-5 

Rate of Growth of Food Production in Relation to Population, 

World and Main Regions, 1952-1962 and 1962-1972 (in percent per year) 











Population Total 


Population Total 









Western Europe 







North America 





















Total developed countries 





















Far East 







Latin America 







Near East